I'm going to log a call on this one, but I thought a heads-up  to the community would be useful.

Basically, if you have a component in an assembly that you've set as an envelope, then Ambien occlusion will cast shadows of the objects behind it even if you set Envelope transparency to 'do not change' in System Options.

It makes for an interesting effect:

Here's the assembly without ambient occlusion turned on (draft or high-quality doesn't matter).  The envelope component is the enclosure itself.

and here's it is with ambient occlusion turned on.