As promised long time ago I am going to do kind of detailed review of my experience using iPhone 3GS and Blackberry Bold. I switched so many times back and forth, so I believe I cannot say that I prefer one to another. I did give up though on Blackberry and currently (and hopefully for a while) on the iPhone, so this is my disclaimer. At the end, this is just my personal experience, so it certainly may be opposite to yours.

So, let's start with the most annoying piece: Reliability and Build Quality.
iPhone had zero problems. Didn't have problems with my old and badly abused iPhone 2G, neither had my wife. No problems with 3GS's. My brother didn't have problems with his iPhone's either. One of my friends got a problem with his iPhone's battery, two days before warranty ended and actually Apple store did replace the phone.
Blackberry gave me some serious hard time. I have used 6 different Blackberry devices in a very short period of time, and 3 (or 4? don't remember really, but it does not matter that much) were brand new, on warranty. They all crashed, rebooted, heated badly (with consequences of being frozen, slowed down, etc.). When it worked it was like a charm. But I actually used "pull off the battery" almost every couple days, and "ctrl+alt+del" used every other day. And you know, it did help. So it reminded me Windows, but heck - I was just thinking it's just a coincidence! Apparently - not. I did dig quite a lot on that, and found out that a lot of people on Crackberry forum were on 2nd or 3rd Blackberry before they found good working phone. Sometimes they had their new phone replaced by refurbished (because these are warranty terms in AT&T) within 2 months.
As for the build quality - I would say iPhone wins hands down. It feels solid and very nicely built, while Blackberry brings some cheap plastic, which is annoying. You can get used to it (comparing to other crap phones like Samsung or whatever else), but it's not iPhone. I do not care about this part too much, but it's just the way it is. Necessary to mention, but it's only up to you if you pay attention to that.
I see the biggest problem here is that RIM is releasing many devices per year, so they know next year you have few devices to choose from, and probably you'll get new one. So there is no need to spend much time and efforts on building nice and solid phone, as customer will get rid of it next year anyway. Apple is not thinking so, that's why you can still see a lot of iPhone 2G around in decent condition, and I wouldn't be alone to say that Apple laptops are living at least twice as much as their PC brothers and sisters :-)

Phone itself. Much happier with Blackberry. It's louder, it heps to dial number faster, you can start typing name and it will go into contacts right away. Very very very much more phone. iPhone still feels like "phone" is just an extra feature among others, not the primary one.

Now, the User Interface. It's very touching and personal, even intimate topic. I happen to like Blackberry UI. It's more serious, if you will, than iPhone's bubbles all over the place. Tired of rounded bubblish icons - but may be it's just me. I did like Blackberry's ability browse through the text with trackball, liked that many things are in text, and that's so easy to navigate. iPhone is very good UI, I just wish I could have customize it? At least some predefined option to make it more "manly"? Or something. It does not affect my productivity with the device, just it would be nice to have something like that.

Keyboard. Like there is much to say here! iPhone has no keyboard. Blackberry has the keyboard. Bold has the best keyboard you can wish for. Very comfortable, very nice feeling, very crazy cool nice keyboard. iPhone has no keyboard -- did I say that already? Yes, I did. But heck, when you are comparing this fact to Blackberry Bold -- you want to say this again. Clearly Blackberry wins for me. And this was couple times a deal breaker for me. It was almost painful to get from Blackberry to iPhone. It literally hurts my fingers ;-)
However, I would not lie -- With landscape keyboard on iPhone you can type really fast. Really comfortable. Relly really an improvement comparing to portrait mode; however, missing real feel is a huge miss for me :-(

Flexibility of Settings. Blackberry wins here again. Very customizable devices. Can do this and that, and also can do this with that. Nevertheless, it failed to support different "From:" for the email accounts (as well as iPhone), and at some point I realized I don't need even half of those customization options. I am stupid, and I am just using what I am using. I don't need too many options. They make me think I missed something. I am pretty fine with what iPhone gives me. Happy that Blackberry does more, sometimes tweaking some minor thing easy makes you feel powerful :-) But I could care less about it. I did like I can use any MP3 track for Blackberry as a ringtone, while for iPhone it's more work. That's the only thing, I guess.
 
Internet and Tethering. Tethering is crazy easy and simple and nice working for Blackberry. iPhone does not have tethering. Well, I remember it did have it in 3.0? But with more recent OS's it does not. You can jailbreak, but I am not going this route.
Internet browsing and Internet apps (like SSH) are much more powerful on iPhone. Everybody knows that, and I am not going to repeat it again. Blackberry, you look like Motorola in 1992 comparing to iPhone for the Internet browsing.
Now, BIS and BES. What the crazy, terrible thing is requirement to have BIS? Why would I need that? Why, no really? Why can't this be an option? RIM is doing this the way -- they are requiring mobile carriers to keep their BISes... weirdo, just another point of single failure (and outages happen) - why would not make it an option? You bring this shit on yourself, so...

E-Mail and IM, and SMSs. I do not use MMSs. They do work on iPhone (finally). But it's not what I care about, so it's not an argument for me. And BBM. I do not use it, and I don't have much friends who are using it. After all, we all can use SMSs, they are not that expensive. And we are using emails. And IM. So BBM -- it's cool, but limit yourself even to big, but still limited Blackberry community - thanks, no.
Now, e-mail and IM and SMS. What I did love that everything goes to one "place" on Blackberry - one folder called "Messages". Yes, there are separate other folders for SMSs, and email accounts, but they all are combined in one. Awesome idea! And the LED indicator -- I can just take a look of my phone and see if I have new email, SMS or IM message... that's worth a lot of me. Really love this, and miss on iPhone. iPhone shows unread counts for every separated entity. Not too useful. Need to remember how many unread emails I had before. Or unread SMS. And I need to "unlock" the phone before I even can understand if I have something new. One tiny indicator would save thousands of my finger swipes :-)
However, the winner is not obvious. Right, Blackberry does push e-mail, but iPhone with MobileMe or GMail (through Exchange configuration) does it too. HTML messages on Blackberry? Save your eyes. iPhone renders them beautifully. Blackberry still does not support just IMAP folders. It does not support two-way sync for GMail (that means if you do something on Web, it does not reflect on your phone). By the way, if you do want to star/archive message from your "Messages" folder - it's not going to happen. To do that you have to go to the separate "email account folder". That's a bummer.

Multitasking. Ahhh, Apple. You suck here. and the iPhone sucks. Because Blackberry's multitasking did work very very very nice. And it did work. And there was multitasking. And, you know, you could never imaging how nice is to have multitasking abilities before you get one (with Blackberry) and then lose them (getting back to iPhone). However, here is my shit to Blackberry's yard. RIM guys. If you do multitasking, why the hell your browser supports only one open page at a time??? What the hell? How crazy is that? My UberTwitter opens page, I want to open another page -- and voila - only one is there. Where are others? Don't know. Neither does Blackberry. There is an option of using Mini Opera 5 beta which has "tabs" feature, but I found it too heavyweight as a browser for my Bold.

Battery Life. Blackberry wins. First, I can change battery. Second, it lasts longer. Only fraction of time, but this fraction is large and powerful enough to show the difference.

Applications. Like Blackberry has them! Just kidding, of course it has, and frankly speaking I am enough satisfied with Blackberry's selection. I did miss couple. Dropbox, online banking apps, Flickr (Blackberry has one, but it's not even close to what iPhone does), SSH.. and games? I do not play them, but for many peope - it's a great "wow" factor.

Media. You can play video and music on your Blackberry. iPhone does it nicer, but still, it's possible on both devices. Comfortably enough. However, when I were in the gym - I couldn't use Blackberry as media device. Unless I just set up playlist and do not touch it. With iPhone I can easily switch/jump/play from song to song, artist to album, and whatever not -- very easily - during working out. UI for media for Blackberry sucks big time, reminds me Windows Media Player.
Photos suck in quality on Bold comparing to 3GS, but "flash" really works nice and in many cases very useful. Video recording - it's cool, but I didn't use much though. And not much sense. With iPhone I can upload my video to Flickr right with app, with Bold it's much more complicated - you either have to email yourself the video in 3GP format, and then do some magic, or you have to download it from your SD card... None of them are natural and simple.
GPS is much snappier on iPhone, it geotags every photo always; comparing to Blackberry when it has to "figure out" something. Using Yelp, Google Maps - those "on the go" apps are much easier and actually bring much value on iPhone, than on Blackberry (besides the latter does not have Yelp :-)

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I guess I covered everything. At least all those moments were important to me, and may be important for you. Ask questions if you have any, I'll try to answer. Would I want Blackberry again? Yes. Or, if given an option of iPhone with keyboard, I would love to have iPhone with keyboard instead. But I guess this not going to happen any time soon :-\ Droid eventually may be an answer, or may be something else, I don't know. We'll see. I am open to what technology can give me.