also make sure you select the transform node of your poly mesh and not the shapeNode before you convert to subD and vice versa, as this make become problematic and create 2 shape nodes (i.e. 2 surfaces get input from one transform node. ) and in the maya doc (5.0... i'm sure it's the samething in 6.0) you can mess with the tesselation of MR (if u are using MR, i assume you are) and MR will not render non-quad subD meshes.
good luck.
i hope the car i'm modeling does not end up 20mb! as my PIII machine would explode! :p