I just installed the most recent version of the wireless driver for Darwin/Mac OS X.

Disconnected the ethernet, plugged in the card, set up the network name and WEP key, and bango, I'm online... WiFi. Unfortunately, since my PowerBook's battery is essentially toast, I can't really use it without being tethered to the wall. (Otherwise I would have set up the WiFi part of my new Linksys router the second it arrived! :-))

So, now I just need the battery.