Before people get all excited: OpenStreetMap barely contains any 3D data. You can get a layout, and guess the height of the houses, if not tagged/stored in OSM, and they usually are not.
Then again, for common elevation data, there are loads of other good sources which can be used in conjunction with OSM, not sure about licensing though.
http://wiki.openstreetmap.org/wiki/Relief_maps
http://wiki.openstreetmap.org/wiki/Contours
http://www.openstreetmap.org/#map=13/33.9147/-112.4034&layer...
granted, this is probably more on the "intellectual property" and "secret sauce" side of things.