Do you need to switch Carriers right now? If not you don't have to restore and unlock yet. Otherwise you might be able to jailbreak 5.1.1, save the SSH blobs, restore to iOS 6, then use the blobs to restore back?
Someone else chime in, but I think that would work.
I hear that if you just go into General > Reset > Reset Network Settings it'll make the unlock "work."
Pop in a T-Mobile sim, I'm not sure where to get them but if you got to a T-Mobile store they might help you check.
Any ideas?? I do have an external drive that is running Lion and has itunes 10.6.X on it. I also downloaded the IPSW for iOS 5.1.1. I just don't know the process I need to go through.
You can restore to a non-current version of iOS. Just don't press the update button in itunes. Turn off wifi or unplug your ethernet cable, itunes won't be able to download the newest update.