Isn’t that implemented for quite some time now?
The Map-settings button opens a small menu which allows you to choose between Google Maps and Marble and depending on that choice to choose the kind of map you want to use. Furthermore you can select the projection for Marble (spherical, Mercator, equirectangular) and you can display a few additional tools like compass and scale on the map.
Marble comes with 11 different views: Atlas, OpenStreetMap, Satellite View, Earth at Night, Historical Map 1689, Moon, Plain Map, Precipitation (December), Precipitation (July), Temperature (December) and Temperature (July).
OpenStreetMap: A global roadmap created by the OpenStreetMap (OSM) project. OSM is an open community which creates free editable maps. The OSM data was rendered using Mapnik.