Metal Gear Solid: Peace Walker, the imminent PSP release in the stealth action series, was until quite shortly before it was announced going to be titled Metal Gear Solid 5.
The Kojima Productions blog (via Kotaku) made the revelation, showing photos of concept logos for the full title Metal Gear Solid 5: Peace Walker.
The number 5 appeared on the teaser site for the game last May. After its unveiling, Kojima called it an "MGS5-class game" and revealed that he was designing and directing it with the MGS4 team.
Peace Walker, offering four-player co-op and compeititive multiplayer with its slice of traditional MGS action, will be released in Europe on 28th May.