Your iPhone does everything else — why shouldn't it unlock your front door, too?
That seems to be the reasoning behind a recent Apple patent application for a "Motion Based Input Selection." Basically, it would fuse a combination lock-style interface with near-field communication (or RFID) technology to act as a kind of security card-key: you'd dial in a three-digit PIN, then wave the phone over a sensor to, say, unlock a door. Or keylessly start your car. Or even pay for purchases on the checkout line.
Mind you, the current iPhone line doesn't support RFID, but much of the world is already enjoying phone-based RFID shopping. Apple's multi-purpose approach (already dubbed the "iKey") could make the concept another facet of the iPhone's must-have cachet.