Hi Mark,
I had a long-running problem with some of the USB devices greyed out and the story of its solution is at:
http://www.linuxquestions.org/questi...ght=usb+greyed
Maybe this is of no help at all, or it may contain an answer.
There's also a very useful VirtualBox forum at
http://forums.virtualbox.org/viewfor...ed8a094e8a6233
Hope one of these does it for you.
Out of interest, did this happen after an upgrade, or has it never worked?
Regards
Girts