T-Mobile and ATT use different 3G frequencies. While the phone can be unlocked and used some functions will not work. Better to stick to phones that are fully compatible IMHO.
Blackberrys are very easy to unlock. Go to FreeMyBlackberry.com. You'll have to wait 24-48 hours for a free code, or you can pay about 3 bucks for an instant one.
T-Mobile and ATT use different 3G frequencies. While the phone can be unlocked and used some functions will not work. Better to stick to phones that are fully compatible IMHO.
Good point, the OP should check carefully before committing to buy.
The biggest problem tends to occur when people are unlocking a 2G compatible Curve and then attempting to use a Three SIM card. For future ref, people need to be aware which frequencies there phones are compatible with, as not all phones will work with all networks.