I really don't know. Given that the form and shape are trademarked, one must worry. But IP law is designed to instill worry, so we need only to worry and not "metaworry." I suspect that if the portrayal of the car is incidental to the photo -- namely, it is only one more object on the street -- then Ford and fellows are unlikely to fuss. Indeed, Disney probably doesn't get permission from Volkswagen* to make Herbie movies -- then again maybe they do. Yuck! More under "book jackets" below. It does seem that trademark protection exists primarily to prevent confusion as to source of origin, rather than to preclude derivative works, and my consuming public, it seems, will be unlikely to confuse me, as the photographer of a car, with an auto manufacturer. Clearly, I'll have to follow the R&R Hall of Fame case to see how this shakes out, since the photographer, though not really pretending to be an architect was sued anyhow.

* I found out recently (fall 1999) that Volkswagen did indeed take considerable interest in the making of the Herbie movies, keeping attorneys on site during the filming. The company apparently feared that improper portrayals of the vehicle might damage its reputation. -- from a video history of the Volkswagen (if it ends up being important for some reason, I can find the exact reference).