This volume will be released on my birthday. ^.^
It is official: CLAMP ran out of RG Veda characters to put on the deluxe covers. Probably realized the fans would kill them if they didn't use Kamui, as well. Pretty picture, anyway. I hope someone gets a full scan of the deluxe cover when the book comes out. Wow, the two Syaorans on the regular cover, that's perfect!
(Meanwhile, still waiting for vol. 11 in English...)
EDIT:
they drew Kamui in the back cover to integrate the vampire theme to this rank,but i would have prefered Kurogane and Fai of course ...but at least it is a good way to prolong the suspense
It's the deluxe cover, not the back cover. Fai and Kurogane have already been on deluxe covers, so naturally it wouldn't be either of them, and the deluxe covers feature only one character at a time.