Update for the Twitter world:
The Vatican says retiring Pope Benedict XVI will send his last tweet as pope around 5 p.m. Rome time (1600 GMT, 11 a.m. EST). That's also about the time he's set to leave the Vatican by helicopter to the papal retreat at Castel Gandolfo.
Vatican spokesman the Rev. Federico Lombardi says the pope's Twitter handle (at)Pontifex will then be "in abeyance" until the next pontiff is selected. He says it'll be up to the next leader of the Catholic Church as to whether or not he will use it. —Joji Sakurai via Twitter
Above: @Pontifex tweets on Feb. 27, 2013.