Keep in mind that if Natal is a success, it won't be an immediate success, and its true potential won't be shown until 2011, when we'll see if the device can provide a sustained lift for the 360.
The Wii wasn't the megahit that it is now the day it came out. The day it came out many were skeptical, and many others didn't even know what it was. It had a great first week because it was the launch of a new console, but that's all it had going for it at the time. It wasn't until Wii Sports got into people's homes, and that word of mouth started to spread, that the Wii took off and had incredible sales throughout the entirety of 2007.
With Natal, it'll probably have a somewhat slow launch, since it is not riding on the back of a new console or anything of the sort. What will make it a success is some sort of Wii Sports like killer-app upon release. It'll sell decently this holiday, with that killer-app garnering great word of mouth, and then if it's a true success, things will take off throughout 2011.
It'll need to time to grow. Things won't happen immediately. This isn't like Halo 3, or any other typical system seller that spikes a console's sales immediately. This is like Mario Kart Wii, and the increase in sales won't happen in a day.