Cheers to this, I say. It means an RPG both I (a PS3 owner) and my sister (a 360 owner) can play without having to lug consoles hither and yon. Third-party exclusives should really end altogether. Between this and Bioshock jumping ship, I think it's obvious Sony and Microsoft have effectively splintered the high-end market, and companies can't limit themselves to half-court anymore.
That said, I'm still glad to have a PS3, not just for MGS4. I like the ability to upscale PS2&1 games, watch downloaded videos off a PC-burned disc, and use USB devices with little-to-no difficulty.
Oh, and Fumito Ueda's a Sony guy, and I'm not missing out on his next project for nothing. Alas, hypocrisy.