So I was wondering.. a Stair or bottom slab is not a valid spawn block for mobs. Although a top slab is. So is an upside down stair also a a valid spawn block for mobs? Like a top slab, it produces a full flat surface along the top surface of that block. But I have never tested this. And it would be useful to know. Anyone know for sure? Please don't post a guess... give a reason.
The upside-down stair is not transparent, i.e. glass, leaves, etc. The upside-down stair is a flat surface with y=0 mod 1, i.e. not slabs, chests, normal stairs, etc. Therefore, upside-down stairs are spawnable.
Minecraft Wiki agrees: "the block directly below it must have a solid top surface (opaque, upside down slabs / stairs and others)" http://minecraft.gamepedia.com/Spawn