From what I remember, it was to be exclusive to the Xbox for a period of time (6 months maybe?) before releasing to the other consoles. But it's been a long time, so please don't ask me for a link to confirm that! Edit--once again, Brian scoops me.
Yeah, six months and online only on the Xbox was the deal, but that deal was made with Interplay for a 5 million dollar loan. But Interply sold Shiny to Infrogrames who either aren't bound by the agreement, or thought it made more business sense to break it (either way MS must have gotten the 5M back).