This week's book giveaway is in the OO, Patterns, UML and Refactoring forum. We're giving away four copies of Refactoring for Software Design Smells: Managing Technical Debt and have Girish Suryanarayana, Ganesh Samarthyam & Tushar Sharma on-line! See this thread for details.
I wouldn't bother trying. It requires correctly catching browser events client-side (and firing off a request to the server) which is a black art, and never works the way you want it to. Easier is to timeout idle sessions.