Check out Trapster.  According to the site..

"When you see a (speed) trap, report it by pressing a button on your phone, or calling a toll free number. Other user’s phones will alert them as they approach the trap. Trapster™ learns the credibility of traps based on how many users agree. It also learns the credibility of each user, over time."

If it works, cool.  But I’d love to see the biz plan, who funded it and what their valuation was.