I've been going through this same debate lately. I've had an iPhone since the first one and a 3GS since launch day. I recently became fed up with network problems I was having with the iPhone and after several calls with AT&T they offered me the opportunity to get another fully subsidized phone. Good on you, AT&T!
So, I went to Best Buy and picked up a curve a couple weeks ago. I have to say it's a great piece of hardware. I've fallen in love with it's form-factor, especially the keyboard. I think the iPhones touch keyboard is great, best around, but it pales in comparison to a real keyboard. I love the full integration of apps, BB & 3rd-party, into the OS. Full-multitasking is a joy.
But, the BB OS has plenty of shortcomings. It is very text oriented and feels kind of old. It's hard for it to compete with the iPhone's UI. Web browsing is much better on the iPhone, as is email in many ways, multi-media, the availability of apps.
I used the BB for a couple weeks and just switched back to the 3GS. I had both working for awhile by switching sims but 3.1.2 seems to have killled that for me. I really wish I could use both. The Curve blows away the iPhone in text input. I write a ton of emails on my phone and the Curve is so much better for text. One-handed typing is a nightmare on the iPhone and a dream on the BB. The iPhone IMO manages contacts and calendars much better than the BB. You can manage separate groups on the iPhone much better. The BB is always trying to integrate them. This was a big reason the iPhone is winning.
For me, it's been a tough decision, one I'm still making, but I think I will end up with the iPhone. I guess I want the complete package. If email and messaging are your main uses for your smartphone, I'd have a hard time not recommending the Blackberry over the iPhone.
I am starting to get bored with the iPhone too. Kind of ready for a change. I just haven't found anything compelling enough to get me to switch and stay.