I had problems upgrading from 5.0.1 to 5.1.1. Ridiculous "iPhone not eligible for firmware update." Despite the hosts file edit and SHSH blob save. It's one of those tedious things you apple users have to do.
Thankfully, I've unlocked my phone perms amenity and saved SHSH through Cydia. I was able to update to 6.0.1 with tethered jb.
Above users have said it all. Latest Redsn0w build can jb 5.1.1 untether, with simple steps to DFU to exploiting Limerain to iPhone.