I have a lot of people hitting my site for info on Revit with Mental Ray Rendering.
This is available in Revit Architecture 2009, Revit Structure 2009 and Revit MEP 2009
Here are a few links that might provide more info…
Mental Ray comparison to Accurender in Revit Architecture 2009 
Revit 2009- two words- Mental Ray! 
Which Autodesk products use the Mental Ray technology? 
3 Comments To "Revit 2009 w/Mental Ray – more"
#1 Comment By Amit On June 2, 2008 @ 6:25 PM
Can u tell me where to download the Mental Ray user guide for RAC 2009
#2 Comment By CADDManager On June 2, 2008 @ 6:49 PM
If you own the product, you can get the books here…
#3 Comment By enermeasedo On December 19, 2008 @ 8:57 PM
Seldom I write comments but resource really cool