A follow up: Make your own USB-based GPIB controller!
http://www.edn.com/contents/images/6290453.pdf
(PDF, 5 pages, 209 KB)
edit: a link to a zipped file (from above article) containing all the necessary info, including sample code:
http://lsd.fe.uni-lj.si/gpib/complete.zip
(1.5 Mb)