Well, for as long as I could remember when you did not have Hardware Acceleration on you could not use ambient shadows (or "occlusion" in other versions). Today, I noticed that in Revit 2013... without hardware acceleration... I was able to turn on and off Ambient Shadows... and they look correct...
|Revit 2012 - Virtual Machine|
|Revit 2013 - Virtual Machine|
Maybe someone in the labs can explain??