Argh. Networks are such a pain. Conference call at 3AM then the network breaks. Traced to an old Linksys WAP54G which wouldn’t pass DHCP addresses. Of course, the Mac doesn’t warn you about this, so signal is strong, but you have to go to Preferences/Network to find out that you are not getting the right IP.
Upgraded the firmware for the WAP54G to 3.0.4 and it starts to work.
But how to see what is really going on with signal. I’ve used istumbler but it only works with 2.4GHz and not with 5GHz. Use Kismac which is way more techie but which tells you about 802.11n and 5GHz too.