Getting your kid to brush their teeth is difficult. Getting them to brush them right is harder. My son and I thought up a iPhone app that guides you around your mouth. Here's a DSi FlipNote of it.
Then lo and behold, Steve of the blog, CamelPunch, comes along and invents the thing. It's a beaut of a project. Based on Arduino. Can't wait to build one myself.
Camelpunch: The Brushduino