I don't know, I have a feeling this is really going to be tough on Sony. The casual motion control games are also very social games. I doubt the pricetag would be below 60 for both parts, so if you want to play 4-player, that would be 240 + the eye.
Wii owners who already have tons of controller/controller parts will not want to buy a whole new set; it's too expensive, especially if games are released for PS3 and Wii. Not even to mention all the clutter from having so many controllers. Universal remotes exist and sell fairly well for this reason.
Also, people who are not gamers and do not own consoles would still typically choose Wii since it will be cheaper overall. The console is cheaper, the games are cheaper, and the controllers will possibly be the same price, or maybe still cheaper on the Wii. Sales numbers for this gen really show that most people don't care about the best graphics so long as the games are fun.
And, I think owners of both PS3 and 360 might (keyword) prefer Natal, even if it were priced over $150, since there will be no extra clutter from extra controllers, and you only need to buy 1 unit. This may be more about preference, but I think some will be influenced by the difference in price, and if Natal is under $100, just makes it all the more tempting to get Natal over Arc.
In the end, Wii is cheaper, and Xbox is cheaper, and while some people are pretty big Sony fans and will stick to them like glue, I'm not sure this will actually bring in many new customers, which I assume is the whole point. I'm mostly talking of people who don't just buy just about everything related to gaming that is. In the end, they need good exclusive software to make this work.
