AHA! Just as I thought! Well... suspected, anyway.
Mythbusters couldn't duplicate the trick. They only tried it on one car though.
http://dsc.discovery.com/tv-shows/mythbusters/videos/tennis-ball-car-unlock-minimyth.htm
Snopes' forum folks said it didn't work on their Altima. http://message.snopes.com/showthread.php?t=2694
Autoblog tried it on 4 cars and it didn't work for them either. http://www.autoblog.com/2007/07/12/can-you-really-open-a-locked-door-with-a-tennis-ball/