T-Mobile's network has never been bigger. In addition to bringing ~1,000 new towers of their own on air every quarter for the last few years, they continue to sign roaming deals, including 850mhz ones.
If you want to see T-Mobile's coverage (down to the street level):
http://compass.t-mobile.com/Default.aspx
(IMO, it's kinda cool how they color code their native network, the 1900mhz roaming coverage they have, and the 850mhz roaming coverage that they have)
For the folks that keep insisting that you only get reception in a building if the network is 850mhz (basically saying that T-Mobile's native network is inherently flawed because it's all 1900mhz), ... you do realize that Cingular has many markets around the country with ONLY 1900mhz coverage (just like T-Mobile), right?