I haven't read all the posts in this thread (the MBP discussions are most of what I'm caught up in now

), but I noticed an interesting article on MacNN that talked about this as well. To paraphrase, it said that when the iTunes TV shows were first made available, Walmart sent back DVDs of Lost, Desperate Housewives, etc., refusing to sell them. Looking on Walmart's site now, though, both Seasons 1 and 2 of Lost are available, both online and in stores (just as an example).
Threatening not to sell movies has a limited impact. I think Walmart is responsible for some 40% of DVD sales (not positive on that number), but if people can't buy DVDs at Walmart, they don't think: "Well shoot, I guess I just won't get this movie." They'll either complain or go somewhere else. This may be tough in communities where Walmart is the only major shopping center, but Walmart can't really blame movie studios when Walmart is the one refusing to sell the product.
I agree with other peoples' posts when they say that Apple's primary movie market will not really affect Walmart's sales (mostly wealthier middle class types who can afford to dump $300 on a streaming media device). I refuse to shop at Walmart for a variety of reasons myself; this just adds to the list.