Yes yes, Gabe has often expressed that the Xbox 360 will be supported. But would you want to see Valve becoming a Xbox 360-only firm? Besides, they have Steam, so they're not in any need of getting bought out.
Epic? They have a 10 years plan for Gears of Wars, exclusively with Microsoft. Where dou you see "switching distributors" there? Sorry, doesn't compute. I do seem to remember though that Mark Reign once mentioned MS would have to offer 2 billions to buy Epic. I think not even MS is that crazy. And why would they? MS is only interested in their own Xbox 360, buying Epic would alienate Unreal Engine for PS3/PC clients most certainly.
Square-Enix isn't big enough to buy Activision-Blizzard. It's more like the other way around, but it seems Activision only cares about Guitar Hero, CoD and the WOW revenues.