Kojima's pissed, reportedly. Apparently a "certain scene" that is "absolutely necessary for MGS" had to be cut from the Japanese version of the game to get CERO's proper certification, which is a C (age 15). You know Hideo Kojima and how much of a perfectionist he is, though I can't picture him with a mad face.
On our side of the pond, a T rating from the ESRB leaves the game fully intact, just as Kojima intended. Same with the UK and its 15 rating from the BBFC. But as the PSP is used by younger gamers in Japan, the cut had to be made there.
Here's what Kojima said on the topic, probably still steaming: "There's tolerance for fantasy games, but for games where you show war or real violence between people, you apparently can't complain about anything. With this, it seems impossible to communicate something through games."
Again, he's pissed.