You really shouldn't read this until you've looked at the video (otherwise you'll spoil the surprise). If you haven't seen the video yet,
go back to look at the video first.
After you've looked at the video, you can read about it at:
http://en.wikipedia.org/wiki/Inattentional_blindness
http://www.wjh.harvard.edu/~cfc/Simons1999.pdf
http://www.physicsforums.com/showthread.php?p=816578
I learned about this visual perception test from an excellent safety talk by Shane Bush, who works with DOE laboratories on safety issues and specializes in Behavior Based Safety, Human Performance Fundamentals, and Human Error Investigation. (For a description of his work, see p.2 of this 2004 newsletter, or this DOE report.) This was part of a mandatory annual safety talk, which could have been very boring, but he was a very engaging and thought provoking speaker.