When I saw them unveil Face Unlock, and then have the demo fail on stage, I immediately wondered why they were even bothering with such a gimmicky feature... and your 'photo imposter' scenario is realer than most people believe [1].

[1] PDF: Paper by Nguyen Minh Duc and Bui Quang Minh from Bach Khoa Internetwork Security Center/Hanoi University of Technology

http://www.blackhat.com/presentations/bh-dc-09/Nguyen/BlackH...