Prototypes and the Vertical Slice

http://news.bbc.co.uk/1/hi/technology/7012688.stm
http://www.gamedev.net/columns/events/gdc2007/article.asp?id=1031
http://sites.google.com/site/teamsketchitup/
http://www.developmag.com/tutorials/31/Grand-Rapids
http://www.developmag.com/tutorials/32/Quick-thinking
http://kimgregson.com/wordpress/2008/11/08/game-sketches-talk-by-john-buchanan-at-future-play/

How to Prototype a Game in Under 7 Days: Tips and Tricks from 4 Grad Students Who Made Over 50 Games in 1 Semester
http://www.gamasutra.com/features/20051026/gabler_01.shtml
http://www.gdcradio.net/2007/01/how_to_prototype_a_game_in_und.html

IGDA Leadership Forum 08 - "Game Prototyping" by Jamie Fristrom
http://video.google.com/videoplay?docid=-4991334930792778473&hl=en

Game Prototyping and New Product Science
http://www.igda.org/leadership/?p=111

Interesting thread with some other professional developers here,
http://www.facebook.com/profile.php?id=635786195&v=feed&story_fbid=90005042144