I am cannibalizing some sections of my wiki to place as entries on this site simply to reorganize some stuff. Here are some links to information about network printer hacking.
Irongeek has a very thorough and well-written series of walk-throughs on playing with networked printers.
Phenoelit [old link] has done some impressive work in the past, which includes their excellent HiJetter tool.
Coincidentally, this same topic just came up on the pen-test mailing list on SecuityFocus. Perhaps some links there will someday be useful.