If there's any game franchise that does its strange, out-of-place product placement right, it's the Metal Gear franchise. From Calorie Mate to Japanese idol magazines to iPods to instant ramen, Metal Gear squeezes it in and makes it feel right, somehow.
Now that magic has been extended to Mountain Dew, with Japan getting a set of eight promotional "Mountain Dew: Peace Walker" cans, decorated with pictures of characters that I do not recognize at all, besides Snake. Players can expect to encounter cans of Mountain Dew in-game as well, though their actual effect is as of yet unstated.
Personally, I hope that drinking the can gives Snake some kind of energy boost, but causes him to burp, potentially disrupting his stealthy action. That would definitely fit the game's style.
Japan Gets Promotional Peace Walker-branded Mountain Dew [Andriasang via Gamepro]