Alert User Before Session Times Out

[eluser]renownedmedia[/eluser]
I'd set it up with AJAX, but ignore the built in session timer, build one yourself. Like, when a "heavy" action is hit, update your expiration time, but when a "soft" action is hit (hitting any page such as the AJAX call) you don't update your timer. Then, when your timer is set to expire in like five mins, your AJAX function would alert() the user.