I'm going to agree with this thread but for a very different reason.
Nintendo has a proven track record with the expanded market. In fact, Nintendo has always tried to make it's games accessible to everyone, but until DS and Wii their hardware wasn't.
Nintendo releases games that grab people's imagination, that are just plain FUN and that anyone can play, pretty much instantly. And most importantly, games that ANYONE, regardless of sex, age or experience with games can and will play. They have 30 years of experience in this field. While another Wii Music is always possible, Nintendo's track record is excellent and we can only expect they'll have something brilliant.
Also remember, Wii's software & hardware teams work TOGETHER. The Vitality sensor was almost certainly developed because the software team had ideas that required that kind of tech. The tech is to support great game(s) ideas.
Sony & especially MS have little experience with the expanded audience that they are now trying to capture. I think Natal is a great innovation. But I highly question MS and Rare's ability to make a game that will grab the imagination of the public abroad, that will be accessible to anyone and will be highly addictive. Yes, Rare makes great games, but not games with mass appeal. Also, their games are being made to suit the hardware, not the other way around.
Sony may have some more experience with the more casual market but they have two huge problems. Arc looks like a Wiimote rip-off and so far it appears they are going to try to use motion in traditional 'core' games moreso than targeting 'casual' games. If that is the route they take, it will massively backfire because people who grew up on dual analog will not suddenly prefer motion (if they did they'd be playing on Wii instead). They will reject it - flat. Especially as it's comes off as a 4 year late wiimote rip-off and not even something unique to the PS3.
While the hardware on both HD systems may be impressive hardware doesn't sell systems. GAMES do. Motion controls didn't sell Wii. Wii Sports sold Wii. I highly doubt Sony & MS will have a true equivalent to Wii Sports. They will have something, but it'll be more a curiosity than a game or system seller.