It depends: I think that virtualbox should actually grab the keyboard (when focused) even when additions are installed if they want to get all the key bindings without making the WM to interact with them (as virt-manager or citrix do); while yes, to make the super key not to be always handled by unity, compiz needs to change (quite a lot) and actually I think this is out of our scope, since it would be a too big refactor and it might break other assumptions we have.
@pabouk:
It depends: I think that virtualbox should actually grab the keyboard (when focused) even when additions are installed if they want to get all the key bindings without making the WM to interact with them (as virt-manager or citrix do); while yes, to make the super key not to be always handled by unity, compiz needs to change (quite a lot) and actually I think this is out of our scope, since it would be a too big refactor and it might break other assumptions we have.