Umm... I would probably stake my reputation on it that the old guy is not big boss. Look at the eye, big boss did kind of lose an eye, also Big Boss would be pretty much decomposed by now. Not to mention why he has a good relationship with otacon based on the trailer, and the colonel... and raiden. Also, Kojima kinda has said you will be playing as solid snake through most of the game, and pretty much all we have seen so far, in fact 99.999% of what we have seen is the old snake. It just doesn't hold together if you watch all the trailers, and all of a sudden look at the old Snake as big boss.
Kojima has fooled before, but I just think it is a baseless rumor, but that does not mean that Big Boss won't be in the game...
BTW, there are a lot of rumors out there, and most are based on nothing and made from people who really don't understand the games.