It's a surprise, but confirmed by a few jp bloggers who figured out what Satsuki Igarashi's 6/11/09 comment was about.
CLAMP illustrated the new Miku Hatsune PV, Mugen no Yami~echo of the past~. The music is by Shinji Hosoe.
You can watch the PV on Youtube or Nico. I'm going to try and confirm this on a news site, but enjoy.
ETA 6/12/09: Nanase Ohkawa's comment today explicitly says that they worked with Shinji Hosoe and the Mugen no Yami video. (you can run their comments through google translate to get the gist of what they say)
Title: Mugen no Yami -echo of the past [original]
Composition: Shinji Hosoe
Song Lyrics: NIMA
Animation work: Mihara Ichirou