One of the external renderers (Lux? Yaf? Can’t remember) has an option to log into the online database from inside blender and use all kinds of presets without ever downloading anything. You’re limited to materials that can be expressed with different settings, e.g. no texture files, but I’d imagine the expansion to allow them wouldn’t be that hard. The point is, the functionality exists within 2.5, the demand just needs to be there for someone to pick it up I suppose.
It was LuxRender. Unfortunately this database is still rather small and lots of materials are missing there.
But LuxRender also includes lots of Volumes presets. Those presets are just (name, IOR, absorb. coeffs) tuples. The database size is probably about 1kb but it makes life much easier. It would be great to have those included with Cycles engine some day.