I understand that highway=path is assumed to be unpaved if located outside of urban areas. I've noticed that this can misinterpret the use of highway=path as a 'universal tag' (http://wiki.openstreetmap.org/wiki/Tag:highway%3Dpath#Usage_as_a_Universal_Tag) where the surface is not specified. In my experience if highway=path is combined with segregated=* then the path is usually a paved cycle (and pedestrian) facility. This is the tagging format used by the JOSM preset for shared facilities.
I hope it may be possible to include such a distinction. In any case, I will keep adding missing surface information where I find it as it is obviously the best solution!