This has happened before. I know Linux doesn't by default have a DNS cache, but once the box tries to use my QWest DSL router as the DNS, it won't properly resolve names any more. The last time this happened, I ended up having to delete my user profile (!) AND wipe and reload my OS in order to fix it.
I know. It sounds impossible. Before wiping and reloading, I logged in as root and renamed my user profile, still had the problem. Put my user profile back in place (/home is on a seperate partition) and wiped the system. Brought the system back up, still had problem. Renamed /home/inthane, logged in, problem went away.
I have no idea what that router does to my system, but whatever it does is fucking nasty. I'm waiting on getting a new DSL router that doesn't have linux compatibility issues before reworking the system.