I read most of the major gaming magazines and am a freqent visitor of videogames.com and there hasn't been any talk of the Metal Gear series on any other platform then PlayStation.
My guess is that Sony smacked Konami around a bit to shut them up until after the PS2 version came out
More likely it was just IGN and Gamespot and other over zealous videogame sites reporting rumors as news. There was never any legitimate or official announcement. Just a lot of fanboy speculation and journalist innuendo. I doubt very much if Sony had anything to do with it as Kojima has repeatedly said in interviews that MGSX's content hadn't been determined.
Konami of Japan announced today that it will develop several high-profile games for the Microsoft Xbox and other next-generation consoles. The major announcement was the confirmation of Metal Gear Solid X for the Xbox. It will be a port of the upcoming Metal Gear Solid 2...
based on fanboy rumors. I'm not saying that this info is still valid, maybe they have changed their minds, maybe there was miscommunication. Or maybe it was true, and for whatever reason they are no longer willing to talk about it.