You can buy an unlocked phone directly from Apple now. It still won't do 3G on T-Mobile though due to the different frequencies used.
yes, obviously. 'unlocked' = 'works on any carrier with which the technology (ie GSM/UMTS, works on the right bands) is compatible'. note that since tmobile almost exclusively uses an unsupported-by-Apple band for 3G (1700MHz), you're getting EDGE at best.