The iPhone Dev Team, a well-known group of iPhone hackers, first discovered the change in an unlocked developer's version of iOS 5, which is expected to be released this fall.
Normally, to restore your iPhone to an earlier version, you'd save "SHSH blobs" (which are like digital signatures to authenticate software) at a specific timestamp and use a third-party app to restore your firmware back to that time. The SHSH blobs are static and can be used as often as you like.
But the team found that in iOS 5, Apple has prevented people from being able to save these blobs for a specific timestamp. Instead, 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.