Here’s a method that I found recently on the buildz blog: Although it is not clear from the rendering dialog, it is easily possible to do a soft-shadowed ambient-occlusion rendering in Revit’s implementation of MentalRay.

This is done by setting the time of the rendering to sometime during the night. After the rendering, just open the exposure settings dialog and increase the exposure until it looks right. The result is below:

Credit for this method goes to David!