While it's not possible to unlock with redsn0w, it is possible to softunlock with white snow or something of the sort.
Jailbreak using the latest Evasi0n software (1.5, I think), then proceed to find Whitesn0w on Cydia. And tbh, I don't even know if this is even updated for iOS 6 yet.
If you don't want to go through the hassle of jailbreaking, just do an IMEI unlock for like, $5.