mm...hmmm... I'm not entirely sure it'd work for /me/, it's like not having Kevin Conroy or Mark Hamill voice Batman or Joker, it's just wrong. Mind you I suppose they could have the guy who voiced Big Boss in MGS4 voice him, that would work as a bridge to MGS4, but it'd still seem really weird. It's like when they suddenly change the actor for a character in a TV show, they can all act like nothings different but it just doesn't click.
If this is the case though, they'd need to get someone with a damn close voice. Mind you, I know it's not /that/ close, but Hugh Jackman would be the guy I'd pick for the MGS movie, but I don't really see him voicing the games.