The Catholic News Agency currently has a story with the terribly misleading and false lede that the "Pope's last Mass will be on Ash Wednesday in St. Peter's". A Catholic news service should, frankly, know better.
The Mass which the Holy Father will celebrate tomorrow in St. Peter's Basilica - as opposed to the usual church of St. Sabina on Ash Wednesday - will not be his last Mass, but his last public Mass. The difference between the two is not insignificant.
Because he is and will remain a priest even after his renouncment of the See of Peter, Benedict will continue to offer the Holy Mass, but in a more private setting, presumably with those will see to his care and consecrated women in the convent to which he will retire.