I remember having this discussion in this thread
http://forum.theiphoneblog.com/iphon...locking-2.html
Since I haven't tried to run an iPhone on the tmobile network I don't know if it works so I would take BadAsh's advice on this and assume there is no 3g for iPhone on tmobile...
But from a technical standpoint (see link above) I understand why it doesn't work.
Tmobile does 1700 2100
Att does 850 1900
iPhone 3g does 850 1900 2100
G1 does 1700 2100
So the iPhone does run on 2100 frequency which Tmobile supports, but maybe they don't really support that yet?
Or maybe you need 1700 for 3g and 2100 is only for 2-2.5G?
Either way just suck it up and move to att, they aren't really that bad
EDIT:
Just looked back at the old post and the link bad ash provided (not sure how i missed it, maybe I saw it preedit?) says the following:
"ATT offers UMTS or 3G using 850mhz(the old TDMA band) and 1900mhz.
While T-mobile offers HSDPA (technically 3.5g part of the UMTS specification) using 1700MHz for uplink and 2100MHz downlink."
So basically you need both 1700 and 2100 for tmobile 3g to function and the iPhone only supports 2100.