The UFC would love to have Fedor, no question.
As I understand it and as referenced above, the issue is that the UFC requires exclusivity in their fighter contracts thereby precluding guys from fighting in competitors' events.
Fedor is, allegedly, unwilling to accept such a restriction to date. However, that being said, this doesn't mean that a deal won't happen but simply that the commercial terms will have to be sufficiently attractive to justify Fedor accepting such an exclusivity restriction (should the UFC's position on exclusivity prove unbendable).
MMA is a relatively new "commercial" sport and the UFC have been impressively quick to unilaterally impose terms on fighters and restrict the fighters' potential revenue streams. I liked the fact that Fitch stood up and was counted (even if he did have to sit back down and sign up in the end after some peace brokering from Lorenzo Fertitta).