Rob Croson
Elio Addict
This linked page has a pretty good explanation of it. "What the mirror really does is flip the order of things in the direction perpendicular to its surface." It also links to a video that I have not yet had the time to watch.
http://physics.stackexchange.com/questions/8227/a-mirror-flips-left-and-right-but-not-up-and-down
http://physics.stackexchange.com/questions/8227/a-mirror-flips-left-and-right-but-not-up-and-down