According to a recently discovered Microsoft patent (via WMPowerUser), it shows a gesture-based unlocking system which sounds simple enough, but how will it protect your phone? Basically it will protect your phone by detecting some very minute details about the way your fingers are positioned on the screen.
From there it will then be able to determine if you are who you say you are. According to the patent, it talks about the angles between your fingers, the timing of your finger when it touches the screen, the size and pressure applied, and so on, all of which are presumably unique to each user. Based on that the phone will know if it is you trying to unlock your device or if it is a stranger.
Now not only will this feature work on mobile devices, but the patent also suggests that it could apply to screens of bigger sizes, and that it might even be used with the Microsoft Kinect. Of course given that this is a patent there’s no telling if such a feature will be implemented but it’s an interesting one.