There was an interesting thread on the Sketching in Hardware mailing list a while back on how to control an inkjet printer. Wendy Ju started the discussion. Following are some of the links from that thread:
A closed, proprietary portable printer: Xyron Design Runner
Nice explanation of how inkjets work and how to control them in general. Link submitted by Scott Minneman
Instructions for now sold-out Parallax Serial Inkjet Kit (which does include inkjet cartridge pinouts for the HP 51604A Black Inkjet Cartridge, as well as board schematics, parts and instruction routines for writing to the cartridge). Even without the device, Parallax’ documentation is handy.
Instructions on how to build your own DNA microarrayer using commercial off-the-shelf parts, including inkjet technologies. The last two links submitted by Wendy.
Haven’t had occasion to use these yet, but they seem handy.