Good suggestions, but I've done these things already. It could be that the signal was (really, really) poor, but I was right beside the Airport basestation (in my lab). And I tested it with a linksys (PC), apple airport (iMac) card, and an Apple Extream card (Power Book). Nothing worked and, if this were the first time, I would just assume that the Airport Extream was faulty. But I'd been through thie whole process once before, with the same results. What was really gauling was that I called tech support who asked for my serial number; they could not find it in their database, so support was refused.
As I said, i just replaced my 801.11b Linksys base station with the new Linksys 801.11g base station and everything work right away. Incidentally, all the computer based 801.11 cards worked perfectly on both the Linksys 801.11b and 801.11g base stations. Point is, this is a real disappointment as just about everything from Apple works without a problem -- including Bluetooth, which I saw others were having problems with.