While on vacation I ran into the issue with getting my AppleTV up and running behind a captive portal. While the AppleTV was getting an IP address from the DHCP server it was not able to connet to the iTunes servers to load any of the apps. This is because there is no browser or method to agree to the terms of service (ToS) via the AppleTV itself.
In order to get around this issue I was able to rely on MAC spoofing. I navigated the menu of the AppleTV to determine its MAC address then disconnected the AppleTV from the Wifi network. I then spoofed the MAC address following the procedure Spoofing MAC Address in OS X.
After which I opened a web browser on my laptop, navigated to the captive portal ToS page and authenticated with the spoofed MAC address. Once authentication was complete I reconnected the AppleTV to the WiFi network and connect to the iTunes servers. I then reset my laptops wireless interface back to its original MAC address so I could continue to maintain access the Internet.
Note you may need to perform this mulitple times during your stay based on the length of the lease for the IP address you obtain.