With all the distros out there I don't see much reason to. Especially since there isn't really anything that "special" about a carPC. At least in terms of the OS. Distributions like Gentoo or DSL make excellent Car distros. Plus, there is nothing keeping you from using the *many* other distributions.
I think what is more important is having CarPC software (GPS, Frontends, wireless manager, etc..)
I could be wrong.