Not necessarily in the sense that you should skip church to raid (though lots of people probably do that anyway). But in the sense that it meets a sociologist's definition of religion: it provides community, ethics, culture, and emotion. And it's hard to argue with that: we're living proof of the community around the game, there's definitely plenty of culture and emotion, and... ethics? CU student Theo Zijderveld is proposing that even if the game itself doesn't promote ethical behavior, the push is there -- we're rewarded for doing the right thing, and often punished for doing wrong. Work with others in a group, get better loot. Camp someone's corpse, and their guildie or alt shows up to camp you.
Intriguing idea, even if it does sound like something cooked up for a college student's thesis (which is in fact what it is). It's certainly not a religion in that there is no higher power involved (unless you believe that Ghostcrawler is in fact a god) -- obviously, we all believe that everything in Azeroth was made by men and women, or at least hard-working Gnomes. But as for what playing World of Warcraft creates in us and makes us feel, those results and ideas are very close in many ways to what organized religion does. Quite a theory.