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Apple looks to be aggressively stepping up its efforts to curb jailbreaking with the next iOS update. According to the unofficial iPhone Dev Team blog, the iOS 5 beta has changed the way the iPhone boots, and the new authentication process will make it harder roll back to older versions of iOS. So, if you jailbreak your iPhone, and something goes wrong, Apple is effectively voiding your Get Out Of Jail Free card.
Once upon a jailbreak, if you ran into trouble and if you saved your blobs correctly, you could use iTunes to restore to a previous firmware version. Starting with iOS 5 and moving forward, everytime you boot your iPhone the authentication process will look at an authentic APTicket, which will be generated randomly. Only Apple will have the keys to sign the APTicket, and previously assigned APTickets will not work. Apple will maintain the ability to turn this functionality on or off. Some tethered jailbreak exploits occur before this authentication process occurs, so for now it appears tethered jailbreaks will remain possible.
The Dev Team considers this "a significant move on Apple's part" to combat, or at least discourage, jailbreaking, but also offers that while "there may still be ways to combat this, a beta period is really not the time or place to discuss them."