Some routers, typically the common windows/mac-centric soho types, are reputed to play a little loose with the real internet dns specifications.

One way to ensure that the router doesn't mess around with dns is to place your isp's primary and backup dns nameservers in the router's setup page. If you don't know your isp's dns addresses, there are alternates like those seen at the bottom of opendns.com

After you put your addresses into the router itself, just restart networking, or reboot your computer.

I tore my hair out for years manually configuring dhclient.conf, dhcp3.conf, resolv.conf etc, to force the use of a real dns address, and not to the little one inside the router. Placing the addresses inside the router's setup page has left me with a few hairs left, and now ALL the boxes on the lans I setup don't have any dns issues.