> BIOS can magically transform keyboards and mice into normal looking
> devices, but as soon as you OS takes over the host controller the magic
> disappears (I would imagine it disappears when the OS takes over
> interrupt vectoring as well).

There are some exceptions about this. Intel USB chip can support
hardware level legacy keyboard emulation for USB keyboard, and
certain NEC's machines sold in Japan use USB keyboard in this
emulation mode. NetBSD can run those machines without modification.
--
soda