I have two examples in Hershey, Pennsylvania of what may be the same thing described in this thread. I have two friends who have home Airport networks with the new Airport base stations. Both have the WAP connected directly to the cable modems. Both lost service in the last few weeks.
When I plug my powerbook directly in to their cable modem via an ethernet cable and enter their dhcp client id I can get a connection fine. When I reattach the Airports after power cycling the base station, nothing. Just a 'self-assigned' IP.
I can't even find any place on the comcast.net support site to ask about this! I think I may have to write an old fashioned letter to the company.