When I get in range, I click the wifi icon in the status bar and choose select connection. It shows me a list of wifi APs, and I choose the one I want and connect. the wifi icon oscillates for a little while, and then it shows the "unable to retrieve ip" error and disconnects.

When it was working, I never had to set up anything to connect to these APs. They just worked. Now they just don't work.

Since it never connects, running ifconfig -a always gives me the same results - no ip address (except the loopback, of course.)