If the phone requires an unlockable baseband...just download Sn0wbreeze:
ih8sn0w.com/.
Then download the 5.1.1 iPSW for your device *model* here:
Firmware - The iPhone Wiki
Choose Expert or General. Either way..Sn0wbreeze creates an iPSW that will not update the baseband, and will jailbreak the phone at the same time.
However...if your iPhone does not require and unlock because you are using it with AT&T anyway....you don't have to worry about preserving the baseband...but if I were you I would preserve it anyway, should you want to sell it in the future. Really increases the value when the device has an older unlockable BB on the newest firmware.