Although its not a Thinking with portals contest they still have prizes, Go check it out http://myapertureinnovations.com/filedb/contest.php > Quote:
First Official mAI Mapping Contest
myApertureInnovations was founded on principals of modding and content creation. As such, our first official contest will be focused around the creative use of Custom Content! Use of custom textures, models, sounds, and particles are all accepted and encouraged in contest entries, so open up hammer and get mapping! The contest will run from Friday, September 21st to Wednesday, October 31st at 11:59 PM UTC. Better get going, you only have 35 days left!
