I'm actually searching for a new phone (i'm on Tmobile) and I was pretty much set on the 3gs (unlocked) until the new Touch came out. Because I am surrounded by wifi (and I don't want to pay for a data plan), I am not going to use the 3g network if i do buy the 3gs.
So now, I can't decide whether to just use my crappy, basic phone and have an ipod touch on the side for all its doohickeys, or get the 3gs. The 3gs will probably cost me $200, depending on my negotiating skills, and I would probably get the $300 Touch.
Even with an unlocked 3G device, you will NOT get 3G connection on T-Mobile. T-Mo uses the 1700 band for 3G while the AT&T uses 2100 band. And since the iPhone was designed for AT&T (and world wide), it does NOT use the 1700 T-Mo 3G band - no way, no how.