I can understand your frustration with indoor coverage on 1900MHz via T-Mobile. If there's an antenna close to your home, then you can get a great signal indoors, but if not then it's not very usable indoors. In CA, NY and NV T-Mobile and Cingular use joint towers so coverage is the same. That agreement pre-dates the AT&T merger and thus doesn't cover the 850MHz towers recently aquired by Cingular from AT&T. So T-Mo users in those states won't get any 850MHz in the deal.
I haven't heard when the iPAQ 6315 might be offered by other US carriers. Hopefully it will be but Cingular might go with the XDA III instead. Your only alternative would be to buy the 6315 without a contract from HP's site or a store like Fry's, then trying to get it unlocked. There are overseas versions (the 6340 and 6365 I believe) which are the same other than they lack the camera. Those are available from importers such as Expansys.