I seem to recall at some point that a glass floor was not a candidate for mob spawning (which would be nice given that one cannot place torches on glass). However, I can't seem to confirm that information at the moment... is ittrue? (i.e. if I have a glass roof, and a glass loft, do I need to worry about somehow keeping it lit)?
No mobs except for spiders (it seems like as of 1.5.1) can spawn on glass. Spiders require a 3x3 area to spawn in. If you place halfslabs so that you have 2x2 cubes of non-opaque blocks and halfslabs around them, they will not spawn. But if you place glass on the same y axis and 2x2 cubes of non-opaque in them, for some reason, they spawn. Don't believe me? Try it. I did it 5 minutes ago.