For the AppleTV
Two options:
AppleTV option, you can set up a DHCP with DNS server address;
To configure your Apple TV to use DHCP, from the main menu, go to Settings > General > Network > Configure TCP/IP. In the Network Setup page, select the "Automatically" option.
Router option, What router are you using? You can specifiy DNS server's in your router then change back to your default after.
For anyone wanting to test the speed of their DNS server and which open source ones would be better, you can use a program called Namebench. You might find that your ISP's DNS server is located in a remote location from where you are and a closer one may be available, making your internet browsing faster and more secure with OpenDNS for example.The only downside is that I have found corporate and some public networks do not allow for custom DNS servers to be used on their networks, a simple removal of them will fix that.
http://code.google.com/p/namebench/