What kind of trouble will I get if I unlocked a iPhone 3GS for T-Mobile?
Legally? None, it's not against the law.
None, just make sure that T-Mobile will support the platform that the phone operates on
install Cydia, u need to know what version of the OS u have, google limera1n and DFU mode, and u need to install ultrasn0w T-mobile supports iPhone, but u are gonna be in edge no 3G because the iPhone has a different frequency and is not compatible with t-mobile. I hope this will help u
Probably the only thing that is bad is if you try updating the phone after the jailbreak. It'll brick it. I have 3 friends who work at apple stores and the genius bars and apple is coming out soon with a software detector that bricks tampered iPhones when plugged in.