Hideo Kojima has announced that Metal Gear Solid 5 is coming as two seperate games. Ground Zeroes is slated for a mid-2014 release, and the core game - The Phantom Pain - is scheduled for a later release.
Here are two quotes from a recent interview with Famitsu:
"The main game –The Phantom Pain – is extremely massive and is still going to take a while. I discussed the situation with the marketing team and after much deliberation decided at least to let people play the prologue."
"It looks like we’ll be able to release The Phantom Pain around the time that the PlayStation 4 becomes widely adapted."