I had the original iPhone and then moved to the 3G in November. Almost a month ago I switched to T-Mobile and the BlackBerry Curve 8900. I don't currently have an iPod Touch, but maybe this will help your decision anyway.
Most of my decision was based on price and usage. AT&T's rollover minutes had me excited initially, but 450 is way too many for me anyway. After one year the minutes expire. I was going over enough on text messaging to warrant upgrading to the 1500 SMS plan, which pushed the bill up higher. With the 3G and an employer discount, I was in the $83-90 range every month.
For me, MyFaves 300 + BlackBerry Internet (with 400 SMS) + student advantage discount (12%) comes out to about $65 a month. The majority of people I was texting are BlackBerry users anyway, so I can get away with fewer texts by using the free BlackBerry Messenger. When evaluating what I liked most and used most frequently with the iPhone in my 23 months of ownership, they were all available on the Curve. The iPhone does some things much better -- media and web-browsing are the biggest advantages, but they weren't worth $25 a month to me (or $17 a month, for those that are doing the ETF math).
I'm also in Chicago, where AT&T's 3G is plentiful. The Curve 8900 is EDGE-only and surprisingly loads Google Maps much faster. My guess is it's a processor thing. The BlackBerry browser and Opera Mini are clunkier than mobile safari, but about equal in speed even with their slower connections. The media player has odd bugs, like not displaying songs in some views, but displaying them in the All Songs view.
I live downtown and have had quite a few calls drop on me with the iPhone. I haven't had a call drop on the Curve, which is at least helped by the Curve's UMA -- a feature that lets you make calls through a wireless router. The signal is always perfect at home, because I'm making calls through my internet connection. The last feature that sold me was the fact that I can tether to my laptop for free. Not only can't you do this with the iPhone right now without hacks, it appears that it will be yet another charge when you can.
Note that over the course of my time with the iPhone I didn't really play games or spend much time on YouTube. It was mainly an email and SMS device for me. The BlackBerry's camera is better, but it's still a phone camera-- which is to say it's nice for when you spot things that just seem unbelievable, like a van delivering freshly prepared food directly to pets.
I may get the next iPod Touch if I'm in a position to devote some time to developing for the platform. Overall the iPhone is an amazing phone and I don't regret the time I used it, nor do I regret my move to BlackBerry.
I think if you're a media-junky or gamer the answer becomes clear; other phones just really can't compete in this area. If not, it is a much more intriguing decision. Do let us know what you decide.