busfault, I believe some of the later PIC uP have USB built in. It would probably be easiest (which may not be what you're looking for to put a usb supporting pic between them. That would probably be the minimum amount of work for you.

You might want to try usb.org for the specification. I have a VERY thourough doc at work that I downloaded at one time. I'll see if I can grab the url off of it...although, I think it was from usb.org. It explains everything.