In my googling I came across I lovely piece of work on rendering lunar eclipses by Theodore Yapo and Barbara Cutler. Here is my favorite part of their results (though not the prettiest-- check the rest out).
Impressive correspondence! Their online talk is a very nice summary of the work, and they have a good figure for the moon not being Lambertian:
So the BRDF of the moon is more like a stopsign than a sheet of paper.