Considering that Forza Motorsport series is the only product that Turn 10 has created for Microsoft thus far, it's probably safe to assume Natal support will work its way into future iterations of the series. Microsoft has already proven that Natal can be utilized in driving games with its Burnout Paradise demo, though its hard to imagine the motion capture peripheral will have the precision for something as technical as Forza.
But who says Natal has to be used for steering? It could be used for secondary activities like shifting, changing the radio station or, most obviously, flipping the bird. We're sure Xbox Live members would just love it.