[harpastum]

Why not just return the visitor to the previous page (if it exists)? If it doesn't exist, Google seems to be the best bet.

[Sidnicious]

I have a better idea (from a usability standpoint): Remove the "I'm at work" button entirely. It serves no purpose (other than being cute the first time it's clicked) so it will never be used. If I'm at work, I'll just close the page or go back.