I just wanted to post a small blurb in here to anyone who's thinking about putting a Treo 750 onto T-Mobile's network. There seems to be some confusion about whether the internet works and all the features of the phone can be set up. The answer is YES!
T-Mobile's customer support was more than willing to help me configure my Treo and swap it in place of my Sidekick3. It took about 15 minutes to put in all the right settings, and my MMS, text messaging, and internet all work like a charm. Having my Treo on T-Mo is kind of like having my cake and getting to eat it too

The only thing I can honestly say I didn't like is that the Treo and a Sidekick3 cannot share a family plan when you need unlimited text on the Treo. The Sidekick data plan plays around with it.