There was an earlier GPIB thread, but it wasn't accepting new replies. Maybe because it was from the old forum site? I dunno. Anyway, I found this and it seems like a good place to start for trying to get GPIB interface on Arduino/AVR. Includes schematics, layouts, and firmware. Will require some updating but not bad.
http://lsd.fe.uni-lj.si/gpib/
The design uses the DS75160/75161 ICs from National Semiconductor. These are not so easy to find, but from what I can tell the Texas Instruments SN7516x are drop-in replacements (?).