Nice to see T Mobile finally offering 3G. If I was to buy and use an unlocked device on T mobile 3G network, what frequencies should the device have in order to make the most of the service? Is Nokia N95 International or US version more compatible. Or is the US version only for AT&T 3G.

I think both will work just fine

Looks like T Mobile runs their 3G band a 1700Mhz which is not common in the U.S.
If you want to take advantage of their 3G service you would need to get a phone from them, at least for the time being until some phone manufacturers come onboard to build some unlocked 3G models with a 1700Mhz radio that the T Mobile network uses.