I had the same issue with my ATV 3 right from the start. I tried several suggestions found searching the web: -restart, restore, update software, unplug cables and wait 30 sec..... etc. Nothing worked.
What did work however was:
Settings > iTunes Store > LOCATION
Needs to be set to "United States"
really not what you would think would matter but it worked (btw I do not live in USA)
hope this may help some of you guys out there