AT&T's 3G operates on 1900MHz and 850MHz. The G1 only supports 1700MHz (used by T-mobile in the US) and 2100MHz (used in non-US countries)

An iPhone 3G (at least the US version) only supports 3G on 850, 1900, and 2100MHz. Therefore, an unlocked iPhone 3G cannot use T-Mobile US 3G service.