If it doesn't come up with "The application Front Row quit unexpectedly" then we may have to assume that something is calling it back out of fullscreen. Do you have an external mouse or keyboard attached, or is anything touching the MacBook keyboard?
If so you may be triggering it to return to fullscreen which it does if a letter is pressed or the mouse clicked - the only keyboard functions it allows are the Function keys, escape, enter, and the arrow keys.
If its not either of the above then I'm afraid I'm rather stumped...