Sorry, I seem to have spoken too soon. Here are the results of a bit more investigation:
- The problem is not with the "wl" driver, because I switched back to the "b43" driver, and the symptoms are exactly the same.
- After booting, with the wired ethernet connected, if I open the Wicd Manager window, it shows the wired connection active and it lists whatever wireless networks are within range. If I then unplug the wired network cable, everything in the Network window disappears - both wired and wireless - and nothing I do brings back the list of wireless networks, it simply says "No Wireless Networks Found".
- If I boot without the wired ethernet connected, the Wicd Manager initially shows whatever wireless networks are in range. But within about 5 minutes, they disappear.
Sorry for the confusion