Members of the iPhone Dev Team saw the change in a recent unlocked developer's version of iOS 5.
For current iPhones/iPod Touch, if you want to restore your device to an earlier firmware, you can use SHSH blobs which timestamp specific times and allow for third-party apps to restore the device.
In the new iOS, the group says "Apple will re-assign your phone a new SHSH blob each time your reboot your device in jailbreak mode, making saved blobs irrelevant since Apple can just reject ones that were saved."