If I'm using a phone with AT&T 3G compability on T-Mobile's network, is it possible for me to use AT&T 3G?

Question

Will I be able to tap into AT&T's 3G if I'm using a AT&T 3G phone with T-Mobile?

Answer 1

No, as 3G roaming agreements haven't been (and probably won't be for quite a while) agreed upon between AT&T and T-Mobile.  

The reason a 3G roaming agreement probably won't come to fruition is because T-Mobile and AT&T use different spectrum frequencies; meaning a AT&T phone won't work on a T-Mobile 3G Network and vice versa.

Even if you are using an AT&T phone with a T-Mobile sim, the sim won't recognize the AT&T 3G service.