SMSC (Standard Microsystems) makes the 97C10x which is an 8051-compatible device with on-chip USB. They created an evaluation board that was basically a PC-AT backplane. It was designed to allow interfacing to 8-bit and 16-bit PC plug-in cards. Following is a link to the board web page:

http://www.smsc.com/main/catalog/evb97c102.html

I played around with one of these boards a few years ago and it was pretty neat. If you are familiar with the memory mapping and port mapping of a PC, you'll be happy to learn that this board uses the exact same addresses as the PC. There's a really simple procedure you must follow to access the "PC" side of the board.

Jon