Flash has some oddities which you have to figure out if you want to have things done with it.
One strange behaviour is what I mentioned in my last post, that is, the Document class (the class on top of the stage, instantiated as root if I'm not mistaken) won't recieve any MouseEvents at all. (If it does, please show me a working example.) A workaround for this can be: place a Sprite on top of the main class (I use the term 'main class' and 'Document class' interchangeably, since they are synonyms) with the same dimensions and assign a listener to the top object. It's easy, and works like a charm.
An other oddity that if you instantiate say a Sprite, it won't have any dimension (well, it's gonna have 0 pixel width and height) until you draw or put some content in it, and then it'll automatically dimensioned to its contents just like a bounding box. (That's why I drew a circle into the main class in my last post - to give dimension to the root -, otherwise how could it receive mouse events if it has no visible surface?)
One other thing I realized that keyboard events will only dispatched to the stage - not any deeper. Say you have several InteractiveObjects on the stage and you'd like to add a listener which keeps track of keyboard events, it simply won't receive any of them. (Okay, probably TextField does, but haven't tried that out yet.) So if, say, you wanted to control a little spaceship on the screen with the keyboard, the keyboard listener must be attached to the stage, and not the Sprite of the spaceship.
My instincts tell me that keyboard events should be passed to all InteractiveObject on the scene, since you cannot focus on one single object like you do with a mouse click - when you focus on the topmost object under the mouse cursor -, again, TextField is/might be an exception. So in this case you'd have two solutions:
KeyboardEvents get posted only to the stage, not any deeper since we don't know which object wants to take care of it
KeyboardEvents get posted to every InteractiveObjects and they'll decide if they want to process them or not
Maybe I should dig a little bit more into the Actionscript 3 documents to figure this out, but so far I haven't had the patience for that, because I have already digested a huge number of documentations and I'd like to get on-field experience already.
I'd like to emphasize that these issues might not necessarily work as I described, only I experienced them this way. If you know answers for any of them, or you have workarounds, I'd be utterly pleased to hear from you in the comments.