This has been going on for a few versions now. Some menus just will not draw or draw incorrectly. I can mouseover and off, it may work, it may not. It is not very random and is easily replicable.
It seems to be mostly gtk apps (screenshots are eclipse, firefox does it too). kde/qt apps seem to be immune.
screenshots - http://img31.imageshack.us/img31/4748/badnvidia.png
xorg.conf - http://pastebin.com/z383xCcw
GeForce GTS 250 / xorg-server-1.11.4 / kde 4.8.0 / linux 3.2.2 64bit / gentoo