Apple is reportedly preparing to ramp up the preventative measures it uses to prevent iOS device owners from restoring older firmware versions, a system often used as part of jailbreak and/or unlocking hacks. As of iOS 5, the iPhone Dev Team reports, Apple will be stepping up the authentication process on the "APTicket" which deems whether a particular firmware version can be installed on an iPhone, iPad or iPod touch or not. Even those owners who have "saved their blobs" - aka keeping a record of SHSH details for each firmware version - so that they can backtrack to earlier, more hack-friendly iterations of iOS will be affected.
The new system Apple is looking to implement is already used in other areas of iOS, namely the BBTickets. Basically, the APTicket will now be uniquely generated each time the user restores, relying not just on the ECID and firmware version of the device, but a random number as well. Each boot will see the device authenticate the APTicket, and as only Apple will have access to the crypto keys, faked or restored APTickets will not be possible.
That way, Apple will be able to shut down the usefulness of older firmware versions, as it can simply turn off APTicket signing for builds it would like to forget. There are some loopholes, such as Geohot's limera1n exploit which works prior to APTicket authentication, and that should mean tethered jailbreaks (where the iPhone must be plugged into your computer every time you want to reboot it) remain possible; similarly, with a legacy version of iTunes it should also be possible to install versions of iOS prior to v.5.
As the iPhone Dev Team says, "it’s always been just 'a matter of time' before Apple started doing this" in response to the ongoing cat & mouse hacking game. They also point to the incoming OTA "delta" updates system as being involved, which will allow for wireless upgrades to the OS with only the newest changes being downloaded. "There may still be ways to combat this" the team concludes, but that will be discussed behind the scenes so as not to tip Apple off to any strategies. iOS 5 is expected to be released this fall.