Gamesindustry.biz has a quote from Senior VP of merchandising at GameStop Bob McKenzie, indicating that it's not just Wrath of the Lich King doing well. These numbers haven't spiked because of all of the returning customers, WoW is gaining plenty of new ones, too. The base WoW package and the World of Warcraft Battle Chest are still doing quite well. Better than last year, he specifies. So while those base package purchases are certainly fueled by the Wrath hype, it's bringing in new customers, not just the returning customers.
WoW may not be the kind of game for every gamer out there, and the game isn't without its flaws. Despite that, it has proven over and over that it's by far one of the best games out there, and definitely the best of its genre. I don't expect it will be going away anytime soon.