[daxelrod]

My understanding is that when you install iOS on an iPhone, an Apple server signs the OS as part of a challenge-response protocol. The challenge includes a unique device ID, and I believe the signed iOS is only installable on a device with that ID. http://www.saurik.com/id/12 has more details.

Think about this in the context of jailbreaking to understand why such a facility exists. Apple doesn't want users to install their own modifications to iOS, and they also don't want users to install old versions of iOS that have vulnerabilities that would allow people to modify the OS.

One way you could implement something like this is to have a public/private keypair within the device and have updates encrypted with the public key; then design the device to only run an OS that it could decrypt with its private key. To do this well, you would need a TPM that did not allow the private key to leave the device, nor to be reset.