While Ars Technica notes that "the patent application itself merely describes a unique way of using motion detection to generate an input, such as turning a virtual combination lock-style dial," the patent itself, as reported by the Telegraph, says that the device could be "any suitable electronic device such as a portable media player, personal data assistant or electronic lock" that could open up any number of physical lock types just by communicating wirelessly.
Electronic key fobs already exist for certain models of cars, most notably the Toyota Prius, which not only allow keyless entry but also allow you to start the car without a traditional metal key. If Apple actually implements this patent and allows iPhones and iPods to act as an "iKey," carrying a ring of metal keys and fobs around in your pocket could eventually seem as passé as a pocketwatch or pager seems today.
While the patent notes that the device would have to be paired with the locks in order to work, and that all communications would be encrypted, people are naturally going to be skeptical about the security of an iKey compared to a traditional metal key. I can see some other potential pitfalls: losing your iPhone, or having it stolen suddenly, means not having access to your car, your house, or anything else accessed with your iKey. Plus, if you're dumb enough to store your access code on your iPhone in a place where a thief can find it easily, it also means that, immediately after finding your home address in Contacts, the thief could gain entry to your house with next to no effort. Or how about this: you come home after a night of carousing at the bar, power up your iPhone to gain access to your front door, but then find a blank screen staring back at you from your iPhone because your battery died.
While the idea sounds great on paper and certainly stokes my science-fiction geek fires, the practical application of the iKey sounds like a giant headache.