WARNING: LONG READ. DON'T EVEN BOTHER JUST SKIP TO THE BOTTOM. I've been following this thing almost religiously so I think I can predict what's happened thus far and how it's going to play out. Ready to eat crow in a week and a half, or not.
Point A) Peace Walker was released in 2010, and since probably a few months after that David Hayter has been at the helm of his directorial debut,
Wolves, which finished shooting at some point after the MGSV GDC trailer two months ago and is currently in post-production. First of all, I don't think Hayter would have had the
time to do any VA work while he was shooting his film. A director (especially one that is working on his first film) needs to be on the set pretty much the whole time it's being filmed, and the fact that it was filmed in Ontario wouldn't make it much easier. After the GDC trailer was released, David Hayter released a very long tweet detailing his feelings about no longer voicing Snake.
It was dated April 1st, 2013.
Point B) When Ground Zeroes was first shown at PAX '12, it was also announced that a Metal Gear Solid movie was officially being produced (as opposed to them trying to hammer out the licensing, it's actually been greenlighted) and that Avi Arad will be producing it. Now, there's been a lot of talk over the years about which actor would be the best fit to play Snake on the big screen, and it can't be ignored that it would largely depend on
which game in the series they were adapting. The actor who would best play Solid Snake circa MGS1 (Christian Bale, obviously) would not be the best actor to play Solid Snake circa MGS2 (I'm gonna say it should be Kurt Russel and that Raiden should be played by Tom Cruise and that it should also just be a shot-for-shot remake of
Vanilla Sky) would not be the best actor to play Naked Snake (Huge Jackman (sorry)) would not be the ideal actor to play Old Snake (probably Clint Eastwood???) I could go on and on.
All that said, Kiefer Sutherland would be the ideal actor to play Solid Snake in an adaptation of
Metal Gear 1 on the MSX, partially because
he can rock a mullet like nobody's business,
but mainly because his dad is Big Boss. Imagine how rad a father/son casting of Solid Snake and Big Boss would be. Are you imagining it? Alright.
Woah, right? Not only that, but fans have been asking for a remake of Metal Gear 1 right now, despite the fact that A) the story is too small and silly to be redone word for word and B) a remake that actually changes MGS canon (ala Twin Snakes) would drive everyone nuts. Kojima also doesn't want to remake Metal Gear 1, he's said as much, and a video-game remake of Metal Gear 1 would be nearly impossible (or just very bad) without him. Approaching it as a movie adaptation makes a bunch more sense. It would also make actually seeing the movie not a redundancy for MGS fans (unlike most game adaptations) since most fans have not played either of the MSX games.
Conclusion) Here is where the big time speculation starts: When Ground Zeroes was first shown at PAX '12, not a single line of english dialogue had been recorded yet. When they showed "the Phantom Pain" at the VGAs, they'd only recorded the lines for that Doctor that looks a lot like Kio Marv, and the woman's (nurse?) voice. Finally, when they were making the MGSV GDC trailer, those were still the only english lines recorded and David Hayter was in Canada working on his film, so Kris Zimmerman (probably?) called up Robert Atkin Downes and a few extras to record Miller's and the two doctor's lines, and got Kiefer Sutherland (who as I've explained is the perfect candidate to play Solid Snake in the movie) to record just a couple throw-away lines for Big Boss and Ishmael, as Hayter couldn't make it to the studio at that point. They knew fans would catch on and decided to turn it into another wacky Kojima prank.
So, my prediction (this last paragraph is a stealth tl;dr) is that Konami will announce that David Hayter is back as Snake, and that Kiefer Sutherland has been cast as Snake in the film adaptation, and that it's about to begin shooting or already has. Who knows, they might even show a teaser for it. Again, this is almost entirely speculation.