The iOS hacking world is full of all kinds of crazy terms and phrases. “Cydia”. “DFU Mode”. “Saving your blobs.”
That last one — the blob one — is a particularly important idea for those on the cutting edge of the jailbreak scene. To break down the concept as simply as I can: by saving a record of a few chunks of data (the SHSH “blob”) uniquely generated for your iPhone during a firmware upgrade, you can later downgrade (read: go back to) that firmware without iTunes throwing a hissy fit. Be it that you ever accidentally update to a build that just doesn’t seem to be jailbreakable, you can return to one that is.
With iOS 5, however, this process seems to be on the way out.
Beginning with the iOS 5 Beta, Apple has changed it so that a bit called the “APTicket” is checked every time the device is booted, rather than only when the device is being restored. The boot process of the device will expect the APTicket to have been recently generated and encrypted (with keys that only Apple has), rendering saved copies of the APTicket (as was done as part of the SHSH blob saving process) pretty much useless. The iOS Dev-Team lays it all out here.
So, what does this mean? Once you’ve upgrade from one build of iOS 5 to another, there’s currently no feasible way to go back to that first build. In theory, you’ll still be able to roll back to pre-iOS 5 builds like iOS 4 or earlier (assuming that you’ve got your “blobs” from those days still saved) — but really, will anyone want to do that?