I believe everyone is blowing this out of proportion, the xbots are jumping with glee not because they get the game but rather because it makes the sony fanboys cry, they do not seem to care for the game all that much to begin with. The sony fanboys are calling foul because they feel betrayed by Kojima for bringing this game to 360 even though it is also coming to PS3 and in the end MGS4 still remains exclusive to the console.
I have a more calm head so I'll try to analyze this, first, we know next to nothing about this game, for all I know it is a FPS starring Raiden using the things Kojima learned from his visit to the US. It could be nothing moe than an experiment on Western Games that may or may not be a good effort.
Second, this game is not stated to be MGS5, it is more like the PSP Portable Ops game, it does have to do with the story but it is not a full fledged game but rather a side story.
Third, in the same way MGS jumps to 360 there might be many exclusives that could also jump to PS3 that we may know by Sony's conference today or even MGS5 so I say people are jumping the gun a little early.
I for one am happy that the 360 owners have a MGS game, that the PS3 crowd will get it too (as well as the PC gamers) and that MGS4 still remains exclusive and is still the best MGS experience yet (even if people that haven't played it or claim to have done so but haven't say that it's a movie with very little gameplay ¬_¬)