Hideo Kojima revealed today at GDC that two previously announced projects, The Phantom Pain and Metal Gear Solid: Ground Zeroes, are in fact the same game. The official title is Metal Gear Solid V: The Phantom Pain.
http://www.polygon.com/2013/3/27/4153444/metal-gear-solid-5-is-the-phantom-pain-and-ground-zeroes-in-one-game
This news is breaking, so expect updates.