Update: Contrary to initial reports, Konami has confirmed that Ground Zeroes and Phantom Pain are indeed separate games. Here's how Kojima himself is describing it:
"Ground Zeroes" is a prologue of "MGSV". 9 years after that event will be "The Phantom Pain". MGSV is constructed w/ prologue and main game "TPP". The game play demo I presented today is the opening of "TPP" which is tutorial that starts from crawling."
Things are never simple when it comes to Kojima.
It has been almost five years since Metal Gear Solid 4: Guns of the Patriots was released back in 2008, and over a year since Metal Gear Solid: Ground Zeroes was announced to the public. So when Hideo Kojima took the stage at GDC in San Francisco today everyone was hoping for new info on not only Ground Zeroes, but also for The Phantom Pain, the mysterious title announced at last year's VGAs that may or may not have anything to do with Kojima's seminal stealth series. And that's exactly what they got.
Taking the stage with his head bandaged to resemble Joakim Mogren - the fictional CEO of Phantom Pain's Moby Dick Studio, Kojima revealed a six minute trailer featuring footage from Metal Gear Solid: Ground Zeroes and The Phantom Pain with the title card as Metal Gear Solid V: The Phantom Pain.
The game picks up with Snake waking up after 9 years in a coma and escaping from an unknown enemy accompanied by a man with a bandaged face. The hospital escape acts as the game's tutorial, but very little gameplay was actually revealed.
No release information was announced, although the demonstration was run on a PC.