To use an HP LaserJet 5si on Mac OS X over the network, you must first use the keypad on the printer to assign it an IP. I chose 192.168.1.2 so it could be next in line after the router. Then in OS X, go to System Preferences-> Print & Fax-> Add new
Click the IP icon at the top and use these settings:

Naturally, you can name it whatever you want. Mine’s in the basement, so that’s it’s name.
All of this couldn’t be more straightforward until you go to print a test page using the default driver OS X selects for you (the HP 5si driver – seems right, yes?). When you print, you’ll get Post Script garbage and tons of blank pages until you quit the job.

To get my printer to work, I needed to use the Gutenprint v5.2.3 driver, which is installed with OS X.

It’s smooth sailing from here and now this beast of a printer can serve me well into the future…living under the stairs in the basement.