Following applies to VBox v3.0.2, with a Windows XP VM on Mac OS-X 10.4.11.
Amagine's solution is elegant and works nicely (mapping a Windows drive to the /Volumes shared folder). I did the same thing first by adding a Network Place, but it operates quite slowly.
What I've now found is to run VBox, but NOT launch your VM yet. Select the VM, go to Settings and go to USB Devices, and make sure USB Devices and USB 2.0 are switched ON. If you have ANY existing USB Flash Drives listed, kill all but one of them. Then EDIT that iyem, and name it just plain vanilla 'USB Flash Drive' - i.e. remove any brand naming etc. Then DELETE any data relating to Vendow, Model number etc in the properties pane for the drive. Click OK, and OK again.
If you DON'T have any USB drives listed, insert drive now and wait for it to mount on the Mac Desktop. Then click the ADD USB Device icon, and VBox should add the USB Flash Drive. Then rename it as above, and delete any 'individuality' vendor data etc as above.
Eject the drive. Start your VM, and once it's running AND HAS FOCUS! then insert your USB drive. Watch the USB icon in the status bar and it should flash and mount the drive etc.
Seems to work for me, but I have still seen a couple of drives that just won't mount at all in the VM. When this happens, mount the drive on the Mac Desktop, and then use Amagine's solution to access the USB drive as a Volume via a shared folder in VBox.
That always seems to work as a good backup.