Yeah, this would be great to have, especially for live previewing of different material combinations. Would require a major rev of the interface/rendering though, so didn't make the cut for launch.
Maybe in a future version then? It seemed like it could be quite easy to sort out with open street maps which I think is already possible to do through canvas.
Perhaps another idea though would be to make your own stylised maps with Mapbox http://mapbox.com/ with OSM so you could print out more stylised maps ;)