Afaik, Metal Gear Solid has never had a Japanese vocal track (though I can't personally comment on the new one, 4)
As far as I know, there was a version of MGS1 with the Japanese voices. And it was on both, a game and a demo of Metal Gear Solid.
Plus, Japan's PlayStation Network store is selling MGS1 in Japanese format. And I agree with Emilio, it does in both the demo and the real game, and I've played the demo of that and I currently own the demo and the real game of MGS4. The demo of MGS4 has japanese voices. Plus, heres a secret; if you set your PlayStation 3 to Japanese you can type and have your character in Metal Gear Online speak Japanese. However, you cannot play against Japan since online mode for america is restricted to america only. But if you want to play against the Japanese either buy the Japanese Metal Gear Online disc or the Japanese MGS4.