Using a microswitch, a webcam, and an A13-OLinuXino. I had to google the last one (never heard of it before now). Turns out its a ARM sysbem that has alot of similar functionality to the RPi. Oh but its about $90 dollars, not including shipping…Anywas, the author has posted the project on github if you want to build your own.
http://olimex.wordpress.com/2013/03/28/make-door-security-logger-with-a13-olinuxino-wifi-opencv/