There's a little ambiguity here, room for confusion. I think people here accept that NPD "cover" Walmart. It's been said here that they do (very large) consumer surveys and estimate sales from Walmart based on the answers given by Walmart customers. That certainly would count as "covering", from NPD perspective especially. But saying they "cover" Walmart doesn't mean they necessarily track them directly.
A game I'm developing with some friends:
www.xnagg.com/zombieasteroids/publish.htm
It is largely a technical exercise but feedback is appreciated.