If you have updated to iOS 6 and you are interested in finding out if you can still downgrade back to iOS 5.x, well I inform you that this is 100% possible. Using the latest version of redsn0w and this tutorial I downgraded from iOS 6 beta 1 to iOS 5.1.1 on my iPhone 4S terminal. In the case of iPhone 4S, iPad 2 and iPad 3, you can only return to iOS 5.1.1 using redsn0w, the rest of the iOS versions being inaccessible even if you have SHSHs saved for them.
In the case of the rest of the iDevices you can use this tutorial to restore to any version of iOS for which you have a saved SHSH, the process theoretically works without problems. I haven't checked on other iDevices, but if it works on the iPhone 4S, it should work on any other iDevice that has a bootrom exploit. I don't know yet if the baseband from iOS 5.1.1 is also downgraded, so don't rush to update to iOS 6 beta 1.
UPDATE: On my iPhone 4S after the downgrade I had the 02.0.12 baseband corresponding to iOS version 5.1.1.
UPDATE 2: It seems that the process can also be done through iTunes, although in the portal dedicated to developers, Apple claims that this is not possible. The baseband downgrade is done now because Apple is signing SHSHs for the iOS 5.1.1 baseband.
