I just think until MS can make some sort of dent in the Japanese market, they're going to be vulnerable. At the moment Sony are so far ahead in every market, they're not so worried about MS.
However, if MS close the gap considerably in Europe and the US next gen, Sony might start to do something about it. They might start to use their influence in Japan to ensure MS get even less big Japanese titles than they have this generation.
I mean, when you look at the big PS2 titles this gen (in the west), there are a hell of a lot of Japanese games there. Gran Turismo, Metal Gear Solid, Final Fantasy, Devil May Cry, Tekken...I could go on. A lot of Japanese games are big sellers in the west, much more so than the other way round. If Sony decided to crack down on these sorts of games being released on other consoles (the degree to which they could do that is debatable), then they'd hurt MS worldwide, not just in Japan.