I think they are being referred to as casual now because of the nature of the WII and DS. Neither are or were meant to be graphically as powerful as the competition, and the games for both seem to be less and less about stories being told or difficulty and more about... well... fun. I myself don't own either but I have a few friends that have never been into gaming (I have always been unfortunate enough to not have any real-life friends that enjoy Final Fantasy. They know how much I love it, I think they see it as a geeky underside that surfaces whenever I can't contain myself and have to talk about some development in the FF series or my personal FF repertoire. I.E. When Advent Children came out they all refused to watch it and I wouldn't stop talking about it, with the result that I found myself being ignored a lot of the time. *sniff)
Sorry about that tangent, my point was that these people barely know what a Playstation or Xbox is, and all of them own a Wii. That would be my personal evidence to support the "casual" comment. At last count, 3 people I know. Make it 4 now, as I've just been told my friends' 60 year old mother bought the 55 year old father one to "keep him fit".
There's no doubt that it's fun, I've played it a few times. Good for pizza and drinks and stuff - but It's not my thing.
Nintendo did a smart thing. The focus on interactive gameplay rather than graphics has made it the new family entertainment console. How long will that hold out for?