To be honest, I think it's a lot more likely that for 'proper' driving games we'll see you using the controller for braking and accelleration, and then you can steer by twisting the controller much like sixaxis allows now, and then maybe with added head-tracking for looking left and right as well as gestures for shifting (again, optional) and handbraking (optional). Something like that. For some things you just need buttons, and you just need to hold something. This is not going to be the area where Natal is going to excell I think - just holding your arms up is exhausting by itself, you'll already need to lean them on your knees or something very soon. Sixaxis steering worked well for me because in the end I needed only very subtle movements and could do it even in just one hand, still using R2 for accelleration (and x for boost). But if Natal could just bring headtracking to a game like Forza 3, that alone would be a big win for me. I really like that technology, especially in a cockpit view.
Sixaxis worked well for me as well in F1 Challenge and Motorstorm 1 (MCs), not so much in Mater´s International. But did I prefer it instead of using the analog knob? No. Not to mention my logitech wheel (in games that support it).
The use of head-tracking to look around sounds like a great idea. I just wonder what it feels like to turn your head but still keep your eyes focused on the same point in front of you.