I too am experiencing this issue with the Layer pulldown, in the Layers Panel of the Ribbon (currently with Civil 3D 2011)... Not sure if this issue also exists in Civil 3D 2012, but I'll be working with that soon as well.
For me, not only does the active layer not show when nothing is selected, but no layer is shown even when a single or multiple entities (of the same layer) are selected.
The only solution that I've come across, is to manually hit the down arrow, and specify a layer. More specifically, using the mouse to activate the drop down menu, and typing "0" for instance, is insufficient. You must manually left click the "0" layer. After that, it (the Layer menu, in the Ribbon) seems to function normally.