In Finland, scientists are developing a cell phone that can recognize who you are by the way you walk. According to one article:
Researchers at the VTT Technical Research Centre of Finland have developed a prototype of a cell phone that uses motion sensors to record a user’s walking pattern of movement, or gait. The device then periodically checks to see that it is still in the possession of its legitimate owner, by measuring the current stride and comparing it against that stored in its memory.
If the phone suspects it has fallen into the wrong hands, it will prompt the user for a password if they attempt to make calls or access its memory.
As for accuracy:
Ailisto says, using the simple motion sensing gait method, the prototype phone correctly identified when it was being carried by someone other than its owner 98% of the time. It also only triggered accidentally, when it was with its rightful owner, 4% of the time.