Funny, I have the Curve, too... but after seeing the iPhone in person, I kind of wish my company would take it away from me
Yep. The only reason I don't have an iPhone is because I get the Curve free through work too.
Not sure about T-Mobile to AT&T, but we transferred her number from VzW with no problems, and before that we transferred it to VzW from T-Mobile. There were a few hiccups on that one, but I don't know if it was because of T-Mobile or Verizon - either way, it was done in less than an hour.
I actually did the pre-qualification at apple.com and entered all of the information there for her old number as well as the credit check, and we were in and out of the apple store in less than 5 minutes (even though they were really busy) with her new phone, and she got a text message before we got home that her number had been successfully transferred.
You just have to give them the existing number, account number, etc. from the T-Mobile account - it's pretty easy.
Thanks for the tip. She ordered the phone online, so if we can't figure it out when it get's here, there is an Apple Sore and an at&t store within 1 mile of our house we can take it to for help.