Though there's little data to back it up, the following conjecture makes a lot of sense: World of Warcraft sops up so much player free time that it's killing demand for other games. Gamers who were previously churning through title after title are now playing WoW (gasp!) monogamously.
Everquest and WoW both certainly had this effect on me and the people who played these MMOs with me. These massive virtual worlds are so immersive and entertaining that they are crowding out (or cannibalizing) some portion of sales of other games. But how much?
Let's take a quick poll: have you stopped buying new games as a result of WoW? Have you avoided a console purchase as a result? How has WoW affected your gaming consumption habits?