There will be a completely different model with a different radio. There are chips that do CDMA and GSM, but those are usually focused on CDMA domestically and GSM abroad. I don't know if those chip support T-Mobile's somewhat non-standard frequency ranges.
Most likely there will be a separate phone for CDMA. Keeping costs down is important, especially since I believe that, at the same price point, the average consumer will have a hard time choosing a product over the iPhone.
In the long run google will probably be competing on something other than price, I'd expect them to roll out a whole pile of android specific service improvements.
Why aren't there chips that support T-Mobile's non-standard frequency ranges in addition to the standard ones? It seems that with the GSM Android phones coming out lately, it's been either one or the other. The Droid Milestone for Telus supports AT&T 3G, but not T-Mobile 3G. Apparently the Nexus One supports T-Mobile 3G, but not AT&T 3G.
