Apple has applied for a patent on a method to differentiate between users of an iOS device. Titled "Systems and Methods for Identifying Unathorized Users of an Electronic Device," the patent describes various ways a device could recognize users, including voice print and heartbeat analysis, facial recognition, and even some form of detection system for "suspicious behavior" (read: jailbreaking, unlocking, hacking, or removing a SIM card).
Such a feature would have many practical applications, not least in security. For instance, an iPhone could lock itself if it detected an unauthorized user and notify the owner by calling or sending an email, not to mention the ability to provide GPS coordinates. It might also be handy on shared devices, like iPads, which are commonly used by more than one person. Different settings and profiles could be applied on the fly for each person.
On the less innocent side of things, Apple is generally recognized as a control freak and you have to wonder whether such a mechanism would be used against jailbreakers. The US Library of Congress recently updated the Digital Millennium Copyright Act with various exemptions, including a few lines that effectively legalize jailbreaking and unlocking cell phones. Apple responded by reminding users that jailbreaking an iPhone voids its warranty.