I'm just about done with wow. Our guild drops Arthas in ~ a month by my estimates (we're almost through Sindragosa). I cancel my account at that point.
I have been very disappointed with LK's raids. TOC was the breaking point for me. I decided to get through Arthas and go. I think a good % of our guild is in the same boat. I'm sure the hardcore guilds will stick around - there's no other game that offers them what they want, but I think the average player is going to leave - for me, the final boss in wow is Arthas - bringing in the dragons and stuff is just a way to keep some new content coming in. He's going to be the last 'human' big bad that we face.
I might come back part way through Cataclysm and join a 10 person casual guild. I might now - my window to play this game is closing - we'll have kids in a couple of years, and I'm not going to play a game like wow once they arrive.