Today, Pope Benedict availed himself of this new possibility and celebrated Mass ad orientem. This is, as far as I am aware, the first time the Holy Father has publicly celebrated Mass in the traditional posture at a freestanding altar which allows for either form of celebration.
Pope Benedict thereby sends an important signal, underlining that this liturgical orientation is acceptable - and even encouraged - not only at altars which are fixed to the wall or to a reredos, and which therefore do not allow for a different manner of celebration, but at any altar where this is physically possible.
Here are some images of the Mass from the L'Osservatore Romano:
