In fairness to OpenMobile, it's implemented using it's own graphics/UI layer. So yeah, it's not going to look or feel like a Mac. But is that a bad thing? There's nothring in MacOS X's user experience that's tailored to the car... unless you run iphone/ipad apps, which you can't (and even those are debatably viable in the car) .
I really don't think it's a great idea to stretch existing desktop paradigms into places where they shouldn't go.