>It's just my understanding. Upper-level object is always here, while lower-level object can disappaer/reappear, so it's up to upper-level to make analysis (at least checking is this object exists or not) and then make an action upon this object.

Hi Ed,
This is an interesting comment, but I see a kink here. What if I have an app with multiple business objects that are mostly dis-similar, but all co-existing together. Sooner or later it's going to be in my best interest to let those business objects talk to each other. In that kind of situation I wouldn't want to depend on my upper level object (which I'm assuming would be my UI or front end tier) to pass the request from one business object to another would I?