http://wiki.secondlife.com/wiki/Mesh/Exporting_a_mesh_from_Maya#Bones_and_rigging
---> "As of viewer Second Life 3.0.0, If you rig a piece of cloth lets say a jacket or a skirt, the "calculate weights" button disappears when you check "skin weights" if you don't at the smooth bind options window unchecked "remove unused influences"..."