A common reason for this is if the DNS information is incorrectly over written by DHCP from the DSL modem/router. (For example, the Actiontec GT701-WG is notorious for doing this.) This can be solved with a simple configuration change.
1. Navigate to the /etc/dhcp3/dhclient.conf file.
2. I recommend making a backup of the file by issuing the following command:
sudo cp dhclient.conf dhclient.conf-bak
3. Edit the file using sudo vi dhclient.conf. Find the line that looks like this:
#prepend domain-name-servers 127.0.0.1;
You need to uncomment the line (remove the hash mark) and replace the 127.0.0.1 with your DNS addresses. It will end up having a format like this with a primary and secondary DNS addresses added:
prepend domain-name-servers 126.96.36.199 188.8.131.52
4. Save the file and restart the computer. (I know you can issue a command to restart the network, but restarting the computer is the real test.) It should behave itself this time, and it works fine for me with Kubuntu 7.10. Note: these changes may not show up if you check the Network Configuration tool, but let your ability to move about on the Internet be your guide regarding whether it can now properly find your DNS addresses or not.