Tobold quotes an interesting observation about raiding which touches on the casual-vs-hardcore debate. I read yesterday that someone will be presenting at the IMGDC about that debate, seemingly to argue that "casual" MMOs are where the money's at.
I generally don't participate in the debate, because a worthwhile discussion is almost impossible when the participants can't agree at least on what they're talking about.
And the only times I find much agreement on the meanings of "casual" and "hardcore", the portrayals of gamers are clearly over-simplified... not quite to the point of being completely useless, but not far from it. The typical agreement among developers is to use available playtime as the measure, but there are often terrible assumptions about how that playtime is spent. Many people seem to think that those with a lot of playtime are all fanatical raiders while those with a little playtime solo and are equally obsessed with achievement.
Amount of playtime doesn't suggest play-style. If a raid takes over an hour to finish, then a person with less than an hour to play consecutively can obviously not participate fully; but that doesn't say anything about that player's desires and what sort of content (apart from time) attracts that player.
If you're going to emphasize time in the definition of "casual" and "hardcore", then don't always use games like Maplestory and Bejeweled to represent "casual" games, because games like Age of Conan and Halo 3 fit the bill just as well.