I was wonder if this was possible and now it has been proved that a user can unlock your device with using Face Unlock..
http://techcrunch.com/2011/11/11/android-facial-unlock-photo/
http://www.youtube.com/watch?feature=player_embedded&v=BwfYSR7HttA
I just tried it with my Nexus and it worked for the one test. I promptly changed it back to my preferred method of security.
Figure this was something for users to know, I have a laptop that has the face unlock feature and I learned that I could do this with it also so I stopped using that feature... I really dont see a method to make that secure enough for me..
It does say when you set up the face unlock that it is less secure than a pattern, pin or password.
That is part of why I went back to one of those methods.
