Insert the following in /etc/fstab

none /proc/bus/usb usbfs devgid=123,devmode=666 0 0

UPDATE:

To get it working on Intrepid Ibex I added this:

#Make USB Work in Sun VirtualBox
none /proc/bus/usb usbfs devgid=46,devmode=664 0 0

Execute:

sudo mount -a

Now you should be okay to run Vbox and use USB devices.