If it were for small button presses, ie. a regular controller, that refresh rate would be a problem, but I doubt Natal will be used to record small fast movements like that. Anything you would want to do with Natal will be picked up fine, and as far as I'm aware it's done on the Natal device itself, so it won't use the 360's processing power (and thus slow down games). What might use more processing power is interpreting the data from Natal, but I doubt that will be too much of a load for the 360 to handle and still have good looking games.
A game I'm developing with some friends:
www.xnagg.com/zombieasteroids/publish.htm
It is largely a technical exercise but feedback is appreciated.