Smooth preview (3-key) render was added to Maya 2009. It is not available in Maya 2008.
Even with smooth preview the model is smoothed at render time to 2 divisions though. It is just done in a buffer so it does not effect the original model. Your options are upgrade to Maya 2009 or greater or put a 2 division smooth on the model and render. You can either keep a duplicate of the unsmoothed mesh or reset the divisions to 0 in the smooth node to get back to the unsmoothed mesh. Just do NOT delete history while the mesh is subdivided.
The mental ray approximation editor for Maya 2008 has problems. Maya 2008 MR approximation editor switched to creating a ccmesh in place of the previous method which caused problems with uv smoothing and textures breaking among other things. It was fixed in Maya 2009 and up but was never addressed in Maya 2008 as far I as I can recall. There may be work-a-rounds but I am not sure how simple they are.
"If I have seen further it is by standing on the shoulders of giants." Sir Isaac Newton, 1675
Last edited by ctbram; 02-10-2011 at 03:56 PM.