I imagine there would be some utility in knowing whether FB employees use your site/app. Though there'd be some complex cross-domain issues to solve first.
Hm, I think you can still invoke it jsonp style and overload the Object constructor in older browsers though, right? This would not only tell you whether they were a FB employee, but perhaps more importantly, their facebook UID without needing them to agree to any OAuth access.
If you check this and it says yes, you'd better hurry up and go to work!