Most iPhone users who have messed up their device by an unsupported jailbreak or even during a routine iOS update gone awry, have been able to rely on the safety net of reverting back to the previous iOS version to make everything kosher again. This practice is known as iOS downgrading and according to developer feedback from new changes in iOS 5 beta, this practice may be on its way out.
Apple appears to be making prudent measures to restrict users from making routine downgrades to previous iOS versions in addition to untethered jailbreaks. By making changes to the “APTicket”, Apple has ensured that certain stages of the boot sequence are now dependent on the authenticity of the APTicket, which is uniquely generated at each restore. Essentially, the device checks itself against this unique ticket number every time it boots up. If the numbers, don’t match, it won’t boot properly. Loading different (jailbroken) or earlier versions of the OS, would cause this number to not match and the phone not to boot.
Now, this wasn’t entirely unexpected and was only a matter of time. It is, however, a big step for Apple in trying to block jailbreaks and towards getting ready to start rolling out future over the air updates. Now, users who have already backed up older iOS version (pre-iOS 5) will be able to restore their phone to the older version, contingent they are using an older version of iTunes.
Before jailbreaking users lose all hope, there are always ways to work around these obstacles after the beta period is over. We’ll just have to see what magic the jailbreaking developers can come up with.