If you watch the new trailer on Gametrailers (which EoFF has a link to on the main page... *wink* *wink* *nudge* *nudge*) there is a scene where she battles Riku at the front gate of Beast's Castle and the Kingdom Key is there. I'm not sure if this is alluding to either Xion being another Keyblade user or Roxas just happens to be knocked out before the scene in question and is just convienently off screen during the scene.

Personally, I bet she's connected to Aqua from Birth by Sleep.