APTicket

APTickets are the new type of SHSH blobs, used by iOS 5.0 and newer. The client (iBSS/LLB/iBoot) generates a random string (nonce), then iTunes and the device sends the request for blob signing and the server returns the data, just like the original SHSH protocol.

Restoring
When iTunes or the on-device firmware upgrader sends a request to Apple's servers to confirm the APTicket, instead of returning the one already stored on the server, a new one is generated. Furthermore, devices with iOS 5 depend on the APTicket being legitimate to be able to boot; the device will enter Recovery Mode if it isn't.

Downgrading
Saurik's original replay attack method of allowing downgrades for any firmware that had been backed up on his server was partially halted for iOS 5 users due to this new system; now it will back up APTickets, but it can't send it directly from his servers; Redsn0w and iFaith can stitch a stock or custom firmware to enable downgrading with APTicket, but it only works for devices vulnerable to Limera1n Exploit.

Faking APTickets is complicated because they are signed with a private key that only Apple knows and they are also partly generated from a random string (nonce).

iOS 4.3.5 and older can still be downgraded if SHSH blobs were saved.

iPad 2 users with iOS 4.3.x SHSH blobs can upload their 4.3.x iBSS via DFU and jump to iOS 5's iBEC. At this point, a signed IPSW can be used to restore to cached iOS 5 blobs. (This method is very effective on WiFi-only models. 3G models require the baseband to be installed/signed... Something like semaphore's TinyCFW can be implemented to get around this).