Here’s an interesting rumour.
According to this interview with voice actor Reuben Langdon, the VA best known for voicing Dante in Devil May Cry, provided the voice of Raiden for Super Smash Bros. Raiden has never appeared in a Smash Bros. game before, meaning he must be referring to the upcoming game in the series slated for release on Wii U and Nintendo 3DS. It is currently unknown whether Raiden will be a playable character or, more likely, an assist trophy or featured in a conversation with Snake.
When asked “Is there a game franchise you’d particularly like to appear in?”, Langdon answered with:
“Let me think about that. Well the Metal Gear series is always coming in and out of my ‘field’. We have put bids out on it a couple of times. Raiden was always a character that I had wanted to play. I did end up playing him on Super Smash Bros; so I did end up playing him eventually. NOT to the extent that he is in the other games.”
It’s possible the actor confused Super Smash Bros. with Playstation All-Stars Battle Royale (a mistake we’ve all made at least once), a game in which Raiden did appear. However, scanning over the game’s credits reveals that Raiden was actually portrayed by Quinton Flynn.
It is possible this is all just a mistake or misunderstanding, so for now it will be filed away under rumour.