Modifier state?

[David Reitter]

Do we have a way to detect the modifier state from the Lisp level, or  
bind commands to modifier state changes?

I'd like to run a command whenever the user presses and releases the  
Meta (not ESC) key.  This would be on non-TTY sessions.

A lot of applications show additional info when you press Alt/Option  
(which is Meta for me); they even update the menus with alternative  
functions.  The user can even view a menu, press and hold Alt/Option  
and see the menu items change.

Thanks
- David