MentalRay just spits none quad subD's back out. The work around is to Subdiv Surffaces > Collapse Hierarchy. It will make the mesh dencer. But will work in MR now. If buy some chance the new subD is to heavy on your render time you can tern down the Tessellation on your shape node. From like 4 to 3 or 2. Will put it ruffly at the same triangle count as befor the Collapse. You probably will want to have the option to work with it more so just undo back past the collapse hierarchy.