Today, the Holy Father celebrated Mass with the members of the International Theological Commission, which has its yearly assembly in these days. The Mass was offered in the Pauline Chapel of the Apostolic Palace, which had been re-inaugurated in July after an extensive restoration work which included a repositioning of the altar so that Mass could be celebrated both versus populum and versus Deum/ad orientem liturgicum.
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: