An exploit in the A4 processor was discovered that makes jailbreaking irreversible by software.
Much like the iPhone 3GS old bootrom hack, a new exploit was discovered in the A4 processor, used in the iPhone, iPod Touch and the iPad, that makes jailbreaking completely irreversible. Since it is an hardware hack, Apple can’t patch it up with a simple software update without updating the entire hardware.
The hack is not convenient to use however, as it requires to be connected to a Mac or a PC when the device is rebooted. Of course, with a large and active hacking community, it will be only days before someone finds a way around that.
This kind of exploit may prompt Apple to issue a hardware revision for its entire range of iDevices. Rumors of it have already started surfacing with MacRumors reporting that a iPhone 4 hardware revision may come by the end of September.