Where has my drop down fullscreen icon gone?

Strange issue,

I can only exit full screen apps with the ESC key, the little fullscreen icon that drops down usually when you go to the top of the screen doesn't appear to be working all of a sudden?

Applies to all my full screen apps.

Anyone come across this and how to get it back ? Thanks.

Reboot seems to have sorted it.... Strange, i guess another little Lion bug