There is a hotsite open for the doujinshi:
The 68 page, B5 format doujinshi will be sold for 1000 yens at the 79th edition of Comiket (Dec. 29~31).
This is the first time in a good while that CLAMP collaborates for a doujinshi.
The news came from a message posted in CLAMP-NET.COM by Nanase Ohkawa.
A little bit of background info now. One of the authors of this doujinshi, artist Kumichi Yoshizuki, is a friend of Ichirou Mihara (from the game developer company ARIKA). As you know, CLAMP and Mihara are close friends. CLAMP and Kumichi Yoshizuki were introduced through Mihara when he visited CLAMP’s studio in February 2009 (exchanged autographs, etc). This collaboration probably sprouted from that meeting =)