Nintendo have the Vitality Sensor, which has even more potential than Natal. I mean think about it, Nintendo have already done everything that Natal is trying to do, Vitality Sensor is completely new and is targeting stress, everybody is stressed! They also have Zelda which is a huge title in America and will be a system seller for sure.
Sony have less to counter right now, but we don't know what'll be out at the end of the year, I think Agent will be BIG but that's just my opinion.. What Sony does have though, is huge exclusives that will push consoles in the first half of the year and thus momentum. Momentum is a hard thing to break and only if Natal really takes off (hardcore and casual) will Sony have anything to worry about.