Remember that April Fools Day video of Metal Gear Solid 4 featuring Altair from Assassin's Creed? If not, here it is.
Well, much like Kojima's previous MGS titles, the costume is real and can actually be used in the final game. It will most likely be a bonus unlockable item or special camo, but it's in there nonetheless.
Remember, MGS4 is just a month away. Personally, I'll be playing in whatever costume Kojima-san wants me to wear. I might even dress up in real life.