Really cool if you've never seen it before. Check the first link for how to do it and then check the second link and scroll down a little bit for an awesome picture to try it out on. Hope you like it 
http://www.starosta.com/3dshowcase/ihelp.html
http://forums.cgsociety.org/showthread.php?f=137&t=779099&highlight=2.5D

http://www.starosta.com/3dshowcase/ihelp.html
http://forums.cgsociety.org/showthread.php?f=137&t=779099&highlight=2.5D