Cannot enter edit mode after moving item to another layer? I just duplicated my model and moved it to a new layer to sort of 'save' a copy of it and now I can't enter edit mode?

You can reproduce this by opening a new project and duplicating the cube via Shift+D and then move that cube to a new layer. Once that happens you're no longer able to enter edit mode. Why? How can I maintain a copy of my model on another layer and edit the current layer?
To enter Edit mode the active Object ( not an : Empty, Camera, Lamp ) should be on a visible layer.