It's of no real interet to me at the momnet (obivously) but this month's
Elektor magazine (the summer double issue, so it's not cheap!) has a
project to make a USB to GPIB interface. It's little more than a
programemd PIC which directly drives the GPIB lines without buffers. I
don't like that much, but...
I think you can get source code for the firmware (apart from the USB
routines, which are standard routins from Microchip).
May be of interest to somebody here....
-tony