There was a video of Kelly being asked about US X Factor a couple of weeks ago and she was very smiley saying she couldn't talk about it.
To me, that says she's signed but can't reveal it yet as she has to wait for the Fox Upfronts.

I would be VERY surprised if Cheryl returned.

I saw that video on E! as well.. Also she seems to be getting a lot of press at the moment as well.