As annouced at Sony press confrence yesterday- another game mgs peacewalker will be on exclusive to the PSP! - the direct sequel to mgs3 (the biggest missing link of mgs franchise).This was annouced along side PSP Go! (e3's worst kept secret)

The game story line is mentioned in the teaser trailer. It also metions a unkown invader (possibly the mysterious mask which has been teased for a week now).