Hiya, I've been testing my game in the Flash IDE.
Everything works fine, I can log in via Facebook and received all details, and have been playing for about 1 hour when I got this message:
got Error: From Facebook: Error: Error validating access token: Session has expired at unix time 1308668400. The current unix time is 1308668443.
As I said, everything works perfectly, I can log in/out save data get friends etc, and this only happened after about an hour of playing while logged in, so it might be a bit hard to reproduce.