Have you tried this suggestion?
I also had this same problem with an iBook and fixed it by retyping my wireless network password in the Airport subpane of the Network system preference.
Well i'm not on an Intel Mac. And my network is using WPA instead of WPA2.
AND, everything was erased and then redone when we made the change (we changed keys at the same time). So effectively doing that - and it's never connected since I changed to WPA from WEP.
Still going insane - at least the kids cant' get online in the morning