In an announcement that surprised no one really, Hideo Kojima announced at GDC that Moby Dick Studio's The Phantom Pain was really Metal Gear Solid 5.
Back at the VGAs last December, one of the title announcements was a mysterious game called The Phantom Pain, developed by new development studio Moby Dick Studios. The announcement was made by Joakim Mogren, whose face was hidden by copious amounts of white bandages. It didn't take long for the Internet to piece together that Joakim was an anagram of "Kojima", and that the character stumbling about in the trailer for The Phantom Pain was the spitting image of Metal Gear's Snake.
In San Francisco, Kojima revealed that The Phantom Pain and Ground Zeroes would both constitute Metal Gear Solid 5. "'Ground Zeroes' is a prologue of 'MGSV'," stated Kojima in a tweet. "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."
Apparently, the reason why Snake's wandering around a burning hospital in The Phantom Pain trailer is that he's just woken up from a 9 year coma.
There was little else revealed in Kojima's conference about Metal Gear Solid V, as the majority of it focused on the FOX Engine and its photo-scanning capabilities.
You can see Metal Gear Solid 5: The Phantom Pain trailer shown at GDC 2013 below.