Was searching for this too. It would be great if it was implemented! :)

I can see one problem with this: You can have a task on multiple Smart Lists, which means that your list would be very long as tasks are listed multiple times. I suppose if you setup your system to manually make sure Smart Lists don't ever overlap, then this would be useful, but I think it'd be a small group that would do that.

Also, you wouldn't be able to use that particular Group By in a Smart List itself.

Maybe I'm wrong, but I can't see this being very useful for most.

This would be a great feature - I would use it to create a dynamic 4 Quadrant list per Covey. By definition, I would have no items appear in multiple smartlists.