Not even sure if there's anything that can be done about this, since Apple keyboards don't have a num lock key--yet I've read that the "clear" key (in the same position) should work as a num-lock key should function on Windows systems as a num-lock key. Regardless; I can't use my numpad on a Windows guest; the numpad only functions as the arrowpad. Using a Logitech windows-oriented keyboard does the same thing.
(Also flip-3d in Vista, which is winkey + tab, doesn't work, but since I've never used it before I can't say for sure that that's somehow VirtualBox or even OS X related. I supposed I'll try installing Vista with Boot Camp and see what results)