Correct me if I'm wrong, but if you jailbreak your iPhone and use it on T-Mobile's network you will only be able to use the EDGE/2G network as the iPhone's 3G radio uses different frequencies than T-Mobile's 3G network.
Makes the choice seem a little backwards then doesn't it?
You're correct, except that it is "unlocking" that allows you to use an AT&T iPhone on another carrier. "Jailbreaking" is completely different - it simply allows you to install unsigned software.
One must jailbreak their iPhone before it can be unlocked, because you need to run special, unsigned software to unlock it.
Makes the choice seem a little backwards then doesn't it?