Not sure - but the only out of the ordinary thing I did in the session was download and install iTunes. Admittedly - I did try and use it before rebooting, but this shouldnt matter, right. Anyway, during reboot the Mac hung with a panel saying something like DragLib -- SteDragImage not found. (I'm at work now, can't remember exactly). After a couple of hours - and invaluable help from this forum wrt Extensions and Conflict Catcher the mac rebooted when I "disabled" something called Macintosh Drag and Drop.
Maybe totally unrelated to iTunes - bit just in case its not thought it worth posting here.