Both St. Peter's Helpers and The Curt Jester have posted pics of Pope Benedict wearing the Camauro, which, according to this website on Roman Catholic Vestments is:
the white fur-trimmed red bonnet reserved to Popes in place of the biretta. No Pope since John XXIII (d. 1963) has used it.
The camauro, like the biretta, evolved from the academic cap of the Middle Ages. Unlike the biretta, however, it did not evolve much. The camauro in its present form was established by the twelfth century.
I'll bet this keeps him warm during the cold winter months. Such style! So appropriate to the season!