Proximity Based Logout [concept]

I was just wondering about displaying a status of whether or not I'm logged in to my own site as an indication of my availability, but the idea is predicated on me not leaving myself logged in to the site when I walk away from the computer. This got me to thinking about a way for the website to know when you've walked away from it so that it can automatically log you out if you get outside a certain range, for example. In order for this to work, the server has to know where you are, kinda. Set it up with an app on your mobi or iPhone (anything GPS enabled, really) and it would seem to be an easy enough concept to submit a list of sites to log you out of whenever you go outside of range. All it would have to do is tell the server to delete your session, on receipt of a msg from your registered device saying that your GPS coordinates have changed in location drastically after having been relatively stationary for an extended period. Give this shit to companies so that they know when their employees are at the bar instead of just the bathroom...