So I'm in the Flash 9 Alpha...
With the new VM there is no need to use MovieClips unless that element has a timeline of it's own.. and there's no need to use Sprite/DisplayObjectContainer unless you're element will contain children of it's own, and so on and so forth.
So I decide in my Ball Throwing example that my ball could be an InteractiveObject just fine, so I extend it and test it, and due to my good fortune I get an Error
ArgumentError: Error #2012: Ball class cannot be instantiated.
So.. I go to Flex Builder and test my code, and it works fine.
So I guess this is just a heads up for you guys to know that this is just an Alpha bug and will be resolved, I sent an email to Adobe.