Hideo Kojima made a guess appearance at Sony's 2009 E3 press conference to announce Metal Gear Solid: Peace Walker for the PlayStation Portable.
Peace Walker takes place 10 years after Metal Gear Solid 3, chronicling the rise of Outer Heaven in Costa Rico. The game is not a side story or spin-off, but a full game written and produced by Kojima himself. The game seems to star Solid Snake, though at one point there were four different versions of Snake on the screen, including a rather disturbing scene featuring 2 Snakes, 1 Box. The trailer featured a great deal of classic Snake sneaking about, so expect this to be more tranditional that say, Guns of the Patriots.
Metal Gear Solid: Peace Walker is due out in 2010. We'll have more info later in the show!