"and everyone has it"
Depending on the price not everyone will have natal. The games will obviously work without it but at the same time developing a game with "natal with" and "natal without" will drive up time/cost of the games.
And the positive of that negative is that the devs will be making double their profit as they will have two markets for their one product
As for dev time driving up the cost of the games that will of course mean that "Gran turismo 5" is going to cost at least $180 right ?.
Good to see all the big devs rating Natal so highly, watched the interviews from the best Japenese devs(they are also on live marketplace) seems they have big plans for it.