Did you know the new mental ray renderer in AutoCAD 2007 will make use of multi-core CPUs or multi processors if present? If you have a multi-core or multi processor system you will notice that during rendering there are two render tiles which indicates each core or CPU is processing a different region of the image.
Animated GIF showing the the two separate render tiles being processed.
You can also change the order in which tiles are processed to even process in a spiral pattern using the Advanced Render Settings Palette.
From the menu View>Render>Advanced Render Settings