manu: It's probably Konami that wants some kind of Xbox presence with Metal Gear. Having MGS4 exclusive seemed to be a generous gesture of Konami's...since they didn't seem to be breathing down Kojima's neck on that. Maybe this game was the catch.
I'm actually expecting MGSR to be the spin-off. Either that or it may have some kind of self-contained canon storyline so that Xbox owners aren't thrown off by story. After all, Kojima has said that he wants to make a game aimed toward western audience (lightning-fast gameplay with Raiden) and one that's aimed toward japanese audience (slower-paced stealth action). I'm looking at Rising to be a series for the Xbox (though it will probably be multiplatform) while the proper installments like the data-heavy MGS4 that have incorporated stealth to stay exclusive to PS3. We may have two games on our hands that will be announced.