Hi,
Can't help you with the keyboard specifically, but I do know a little about the PS2 protocol. Look in the arduino playground for the PS2 mouse interface sketch. The same information applies to keyboards. So most of the code (except for the stuff in the main loop() routine) should apply.
Unfortunately, you can't just use the keyboard as a big mesh switch panel. It doesn't work that way. The keyboard sends scan codes, which must be decoded into characters or key presses. If you are just going to use a sub-set of the keys, then I think a sketch to do that might fit into the arduino memory.
See this page http://www.computer-engineering.org/index.php?title=Main_Page
for info about PS2 keyboards. It might get you started.
Good luck,