Okay, so I wasn't intending to start yet, but I just happened to check the Anime On DVD forums and found out...who the Tsubasa - Chronicle seiyuu are!
Syaoran (Irino Miyu)
Sakura (Makino Yui)
Kurogane (Inada Tetsu)
Fai (Namikawa Daisuke)
Mokona (Kikuchi Mika)
(posted by littleharlock. I think the information has also been added to the Anime News Network database, it shows up on the seiyuu pages but isn't showing up on the series page yet).
I'm a little bit surprised - I only recognised one name (Irino Miyu - Haku from Sen to Chihiro no Kamikakushi, Daisuke from D.N.Angel), and sure enough, I haven't seen many other series which the others have done. Fai's seiyuu should be familiar to some - he's in BECK and in GANTZ...maybe I should start watching the BECK episodes I've collected so far!! But for Syaoran...I loved Irino-san as Haku, but despised him as Daisuke, so what will happen with Tsubasa?
April cannot come soon enough!