I had this problem, and it took me an unusually long time to fix. I believe the cause is that the network we use here at home changed settings. We changed from WPA to WEP, or something like that. So even though I modified the settings in the Network Preferences Pane to WEP, it still didn’t work. AirPort would say it had no IP address, or a Self-Assigned IP address. And I had no internet connectivity.