From ArsTechnica:

Those who screw up their iPhones with an unsupported jailbreak—or who have bad luck with their iOS upgrades and want to roll back to an older version—are usually able to make everything right again by downgrading their phone's OS back to the previous version.

That practice appears to be on the endangered list, however, thanks to changes within iOS 5. A post at the Dev-Team Blog indicates that Apple is taking steps to inhibit downgrades to previous versions of iOS, in addition to untethered jailbreaks.

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