Can i use a AT&T iPhone 4 if it is reported stolen?

An i use a AT&T iPhone 4 if it is reported stolen?
i traded my iPhone for a white iPhone 4. And the guy said he gonna report it stolen, if you took out the sim card. Can i unlock it and use it on t-mobile still?

Why would he do that?
Once the phone is reported stolen and you have it, they will take it from you and won't return it back. You can unlock it but that doesn't change the serial number for the phone since that's how all companies keep trap of their customers phone and information.

Probably no, You should ask this service provider http://trustediphoneunlocker.com about this issue.