Hi, do I understand you correctly? I am hearing from that if I unlock the iPhone and then switch to T-Mobile, I will have voice service and data service. However, the data service will be slower than I would get had I stayed with ATT? thanks.
That is correct. AT&T's 3G service operates in the 2100MHz range and T-Mobile's 3G service in the USA operates in the 1700MHz range. The 3G radio in the iPhone is incapable of working in the 1700MHz range.
However, both AT&T and T-Mobile use the same frequencies for EDGE and you should be able to get by with EDGE service from T-Mobile as a result.