The folks over at OSXDaily are reporting that, according to the jailbreak experts at the iPhone Dev Team, iOS 5 will effectively prevent you from downgrading to lower versions of iOS.
The change is said to be the result of certain subtle changes in the boot sequence of the latest iOS 5 beta, and will only affect restores from iOS 5 forward. Interestingly, they also note that Apple will be able to turn this restriction on and off at will. They also suggest that it may be possible to find a way around this latest restriction.
Note: This change is NOT live yet, so users of iOS 5 beta 2 can still downgrade to iOS 4.3.3 if they so desire, using the instructions we provided earlier.
The iPhone Dev Team hasn’t released much information about this restriction.