Thanks so much MoonMind. I had also found some troubleshooting info
here, among other places.
Eventually I solved the problem by installing Jaunty (Ubuntu 9.04). I believe in this case it is better to make a fresh install rather than upgrade Intrepid to Jaunty, for two reasons:
1. In Intrepid (8.10) there was some "blacklisting" of the ath5k driver (for the Atheros wifi adapter) that might carry over if you upgrade from a working installation of Intrepid to Jaunty.
2. Booting from a live CD of Jaunty will at least give some indication whether or not the device is supported - i.e. if it's not working in the live session, it still might work under the installed system, but if it does work in the live session (as it did for me) then you know it has to work after you install.
At any rate, thanks again, problem resolved