If you look at the view counter on that forum's topic list page, this issue is really popular.
If you plan to use group policies (as almost every Windows admin does) you have to move your user and computer objects into organizational units, because linking a GPO to a container (like CN=Users) is not possible.
If you want to use openchange on Zentyal you are forced to put your users into that CN=Users container which in turn does not allow you to use group policies for your users at all.
So you must decide if you want to have group policies OR openchange... This is completely crazy!
Why isn't anyone of the development staff answering this topic?
If this restriction is not removed soon, openchange is completely useless as a Microsoft Exchange replacement!
Just my opinion
EDIT: Can you please explain in detail where the TECHNICAL limitation is?