When I first got mypre previous iPhone it was unlocked and i was using it on a
tmobile. When I connected it to iTunes and updated it by accident it locked it back to AT&T. So updates will relock and unjailbreak your phone. I do know anyone that updated to 3.0 from 2.2.1 on an unlocked iPhone so I'm not sure what will happen with that but with the other software versions it relocked it.