Sony clearly have a deal with SE to keep it exclusive in Japan. I'm sure Square Enix wants this game out as soon as possible, a 360 version will only delay things.
I don't think Microsoft is petty enough to pay 40 million dollars to make it multiplat in Japan just to spite Sony. They get some 6 bucks per game sold, so with the 400k this would probably sell on the 360 in Japan, they'll get 3 million back.
Contrary to what some people believe, Microsoft are not business morons.