The iPhone Dev-Team has posted following the release of iOS 6 to detail which devices can be downgraded from iOS 6.x to iOS 5.x and which devices can be re-restored.
A few months back we released a redsn0w feature that lets you downgrade A5+ devices from 5.1.1 to anything lower (if you had saved blobs). Unfortunately once the 5.1.1 window closes, *redsn0w's 5.x downgrade feature will no longer work.* Most A5 users will not be able to downgrade. So if you're an A5+ owner up at 6.0 when the 5.1.1 window closes, you'll be stuck there without a jailbreak for now.
We're happy to report there are some serious deficiencies in the 5.x restore process that are permanently exploitable. They'll never be fixable by Apple because they're all self-contained in the 5.x IPSWs.
Here's the breakdown:
● A4 devices and 3GS will always be downgradable (and jailbreakable) due to limera1n. The tethered iOS6 jailbreak for those devices (and untethered for old-bootrom 3GS) will be out when Cydia and other important pieces are all working properly
● iPad2 owners who have both 4.x blobs and 5.x blobs will always be able to downgrade to those versions, even once you come up to 6.0 and the 5.1.1 window closes (don't do that yet though!). You need both 4.x and 5.x blobs to qualify for the downgrade even if you only wish to downgrade to 5.x
● iPad3, i4S (and iPad2 owners who don't satisfy #2) will always be able to RE-restore the current 5.x OS that's already on their device. So if you're at 5.1.1 when the window closes (and you've saved your blobs), you'll always be able to RE-restore to 5.1.1 again. This makes the 5.1.1 jailbreak a lot less fragile - you don't have to worry about messing up your install with funky extensions or getting into a boot loop, because you can always RE-restore from 5.1.1 to 5.1.1 again (or from 5.0.1 to 5.0.1 again, etc). But once you fall off the 5.x train by restoring to 6.x, you'll be stuck there until the next jailbreak.
Notably, re-restores and iPad 2 downgrades will leave you with the latest baseband so don't do this if you need an unlock.
The iPhone Dev-Team will release these new features in an update to RedSn0w that is coming shortly. In the meanwhile please backup your iOS 5.1.1 SHSH Blobs using one of the tutorials below.