Ever since the release of the first beta versions of iOS 5, Apple has warned developers about the possibility of reverting to an old version of iOS, and in iOS 5 the warning is still present. Apple claims that if we update to iOS 5.1 we will not be able to return to an old version of iOS. Well, things are not quite like that because a restore to an old version of iOS is possible from DFU Mode and if you have SHSH you should have no problems restoring even to iOS 4.x, regardless of the version.
If you need unlock and jailbreak, I recommend you not to bother with installing iOS 5.1 on your terminals and to wait until the final version is public and there are jailbreak and decoding solutions for it.
