I still read stuff on my old guild forum from time to time, and at least once a month there's a person quitting to "get his life back together" - and I kid you not.
I'm not sure what that means in terms of statistics, but it tells me that the problem is a lot more serious than what some of you are trying to claim.
One thing you'll find with many "addicts" is that they're not aware of it until it's too late. They go on about it not having an effect, and they're just playing instead of watching TV or drinking with their buddies. That may be true, but in my world playing one single game for so many hours at a time is a VERY different thing from doing a VARIETY of other things that involve reality.
I was "addicted" myself for a long while, and it was especially harmful to me because I was sharing this addiction with my girlfriend - which essentially meant we both neglected a lot of stuff. No, it wasn't like drug addiction and today I suffer no real injuries as a result, except I have ~180 days of my life less to spend on other things. I enjoyed WoW for sure, but the game was basically over within the first 6 months - and I regret not spending time doing other things. It's one of the most blatant wastes of time I can think of, and this new "achievement" approach to WoW is downright insulting. Such a system, once in place, is about the most hollow waste of time that I can imagine. Yet people chase these little numbers and titles like there's no tomorrow.
As a game designer, I bet you I could make 100 achievements a day without even setting my brain in motion. It's scary that Blizzard have become OK with adding "content" in this way.
I got out if it a lot less "damaged" than several I know on a personal basis, and in fact I still know people who dedicate their lives to this online fantasy world.
They'll say it's not a problem, and maybe it isn't - but that's not what I'm seeing that's for sure.