Earlier last year Konami had been hinting at a big surprise for Metal Gear Solid: Peace Walker, which has still yet to be revealed. According to a recent post on the Kojima Productions blog, it appears the company might be getting ready to reveal that surprise.
Metal Gear Solid: Peace Walker takes place following the events of Metal Gear Solid 3: Snake Eater and Metal Gear Solid: Portable Ops. The story revolves around an unknown military force equipped with the latest weapons which begins to take over Costa Rica. The country's constitution bans the creation of an armed force, so they secretly hire the group which Snake is involved in to turn back the threat, which could shift the balance of power between the East and West.
In the blog post, Kojima Productions PR manager Jiro Oishi says the following after talking about touring Japan to show off the co-op play in Peace Walker: "However, in truth, there's another ultimate charm outside of co-op play hidden in PW.". The original birth of this news came from Hideo Kojima himself, so it is likely that Oishi and Kojima are both speaking about the same unannounced feature.