One big advantage for iOS users who jailbreak their devices is that they can downgrade their firmware if they come across issues in a new version of iOS. According to a report from the Dev-Team Apple has been working on a way to block jailbreak users from restoring older versions of iOSTo restore to an older version of iOS is done by saving your SHSH blobs of the older versions on Cydia servers. When you are restoring this allows your device to check with Cydia instead of Apple.
Since Apple's servers don't allow restoring to older versions by using iTunes by default, restoring through Cydia was the only way until now.
Apple is planning to put a stop to this method with the release of iOS 5and this process has been revealed in beta versions of the iOS 5 software update. Your device which has iOS 5 installed will check it's 'APTicket' with each boot. Since the APTicket is uniquely generated each time the device is restored you cannot use the existing method to save the SHSH blobs.
At this point the Geohot - Limera1n jailbreak should not be affected as the APTicket check takes place after the jailbreak takes effect, so this should not affect an untethered jailbreak in future.
source - Dev-Team Blog