“Their (Wal-mart merchants & little known employees) decisions about what makes it onto Wal-Mart’s shelves have enormous impact, earning (or costing) vendors millions of dollars. It was a blogger on the Check Out, after all, who first disclosed last month that Wal-Mart would stock only high-definition DVDs and players using the Blu-ray format, rather than the rival HD DVD system. The decision was considered the death knell for HD DVD.

usebytes.com/tech/2008/03/03/ny-times-says-wal-mart-killed-hd-dvd/

[Unlinked URL – Tony]

I wouldn't say Walmart was the nail in the coffin. Best Buy dropped support; Netflix dropped support; Warner Bros. dropped support all within a few weeks of each other.