Can Moss Stone (Mossy Cobblestone) be found in SMP?
Yes, Moss Stone occurs naturally in SMP. It can only be found on dungeons walls. The quantity varies, but my friend got around 55 blocks with only one dungeon, playing in SMP of Beta 1.5. The minimum seems to be 30 blocks, with a maximum of 64 per dungeon.
Dungeon, with part of the floor composed of moss stone:
Source of some information: http://www.minecraftwiki.net/wiki/Moss_Stone