That 8.6 million figure of US retail sales for Blizzard's MMO includes the original game, it's two expansions and special Collector's Edition and the Battlechest bundle.
In retail sales alone the game World of Warcraft has raked in around $258 million from US consumers. Now just add the $15 a month subscription charge... no sign of slowing.
These figures have been gleamed from NPD number crunching. Most recently Blizzard has experienced some trouble with changing operator in China, but it's likely safe to assume their 11.5 million subscriber milestone will be fighting fit again with its relaunch.
Can anything stop World of Warcraft from dominating the MMO space? There's very few contenders out there... maybe BioWare's fully-voiced Star Wars: The Old Republic?