That doesn't work in most environments. In our offices, our input devices, printers, scanners, and other equipment are all USB or Firewire, so locking out USB from non-admins essentially makes the computer nonfunctional. Connecting a USB device is merely a matter of unplugging the printer and plugging in your USB fob. I have a 5Gb USB flash drive on my keychain and could copy most of our client data in about two minutes if I were so inclined.
Since most HW manufacturers are moving away from PS/2 as a viable plug option for input devices, this problem is going to get far bigger.
Agreed on the ps2, but everything is networked here so it works for now.