Original Post: http://blog.nvisiumsecurity.com/2011/04/exploitable-mobile-app-challenge-now.html

You can read the details on the above link, but it boils down to you make an application for iPhone or Android. You make it vulnerable to X,Y,Z types of flaws, you win a 32gb iPad or a Motorola Xoom.

Added bonus, all the apps get submitted to OWASP for people to learn Mobile security.