Someone's already made this six months ago on the iPhone: http://www.knocktounlock.com/ http://www.knocktounlock.com/ I've been using it for the past 6 months

Someone's already made this six months ago on the iPhone: http://www.knocktounlock.com/ http://www.knocktounlock.com/ I've been using it for the past 6 months and it went from futuristic novelty to now daily use.