I think people don't realise that the announcement won't be considered big if it's stealing another exclusive from Sony, mainly because no one really cares anymore as almost every big third-party games are available on both platforms, so I doubt it would shock anyone to see MGS4/5 or FFvXIII on Xbox 360, because PS3 also got some of the big Xbox 360 third-party exclusives like Ninja Gaiden 2, Tales of Vesperia, Bioshock, Lost Planet...etc.
What I think will be surprising is to see Microsoft securing an exclusivity of an expected multiplatform title, like seeing MS announce the next big Valve title for Xbox 360, or securing Modern Warfare 2 exclusivity (timed or extra DLCs), or the next Level-5 project, or the next Bethesda Softworks title...etc.
Anyone really would be shocked to see Versus XIII on Xbox 360 after seeing XIII heading over to Xbox 360? Or seeing MGS4 port after a year and a half? I really don't think these would do anything more than fueling fanboys, but I guess Microsoft really cares about fueling fanboys... So anything can happen actually. -_-