Looks like Microsoft is getting somewhere with WGA. They have recently filed lawsuits against some of their resellers for supposedly selling stolen or pirated software. It's a relief to see Microsoft isn't taking the obnoxious RIAA route and going after individual consumers, though I wouldn't hold it above them to do so in the future. The original article is light on details, but supposedly most of the information was gathered via a secret shopper system and not WGA, though WGA may have been one way they were tipped off to particular resellers. They have done this in the past, and likely will do so again:

Mary Jo Schrade, a senior attorney at Microsoft said that "our message should be made very clear by today's lawsuits. We are committed to finding the unscrupulous dealers of pirated software and making piracy a business model that doesn't work."
After purchasing systems anonymously from the resellers, they would then test the systems to see if they had legal copies of Windows and possibly other software like Office installed.