If you run into Error -15000 or Error -15006 when using iTunes to stream to your AppleTV, you should try turning off IPv6. Whether on WiFi or Ethernet, it doesn't matter. Use the following two commands in your terminal to turn off IPv6 as Mac OSX 10.7 (Lion) nor 10.8 (Mountain Lion) give you this option in the Network TCP/IP settings.
networksetup -listallnetworkservices # this will list out the network interface names
networksetup -setv6off Ethernet # or Wi-Fi ... this will turn off IPv6 for the interface