On impulse, I drove to the computer store today and picked up a Cisco 350 series wireless networking card. It appears to work fine with Linux. I'm currently participating in an ad-hoc wireless network relayed through a laptop with two wireless interfaces--one to serve the local net, and a long-range card to relay traffic to a nearby building. Slick.
I still don't have a particularly good grasp on this technology--there's a lot of strange parameters and configuration options, especially under Linux. But it works.