I see Mapnik now renders highway=unsurfaced and highway=raceway. (Update 1-aug-2009: corrected tags.)

The former works out pretty well since I often want to know whether I can ride my road bike on it. I think of highway=track as a dirt road.

The latter isn't really all that important, but it helped me with Portland International Raceway. Initially, I took a look at either Hockenheim or the Nurbringring. They had the track labelled access=permissive and oneway=yes. It kind of made sense, although it seemed like it could also be access=private. That said, a separate tag for a racetrack just feels like a better solution.