As far as I know, Steve Purcell did story development, storyboarding, and other concept work on the film. You can see what looks like some of his art in the storyboard stills/concept work that's been released. For instance this sketch
looks pretty Purcell (check out the Route 66 sign). I also predict we'll be seeing him mentioned on the Cars DVD special features in some capacity. I hope so at least.