Hopefully Square Enix does get their act together and start developing for M$ consoles, though I could care less about the games they produce. We just need more power to the Xbox(s).
However, a lot would have to go awry for them to completely leave Sony. I think either way they will still work with Sony, which means that they will make games for Sony, and people who are already loyal to Sony (most of gaming population over there) will just continue to play those Square Enix games on their already owned Playstation consoles, and won't have the need to "waste" their money on the Xbox. Unless SE completely leaves Sony, Xbox will still be in the same basic position as it was before.
(This is all assuming that Square Enix doesn't come out with a jaw dropping revolution of a game just for the Xbox(2), which I've yet to see them do on any console.)