Almost 2 years ago, I was dreaming about this series (manga) getting animated at some point. Now that it's about to start production, I'm getting more and more apprehensive about it because there's relatively high probability of them messing it up. It's a delicate and wonderful series, after all.
Anyway, I do not know if they've already decided on the casting for the anime, or if they even had an audition, but being a seiyuu DD, I wanted to put in an updated guess before it's too late.
Before, I said Shizuru could be voiced by Chiba Saeko. That was just a whimsical statement. I put in some more serious thought into it and came up with the following.
For the role of Shizuru, Yuzuki Ryouka would be a great fit. She has to be externally calm, yet always thinking about things internally. I think she'd be great for this role. And for Mizuki, my #1 pick is Orikasa Fumiko. Her spunky kid voice would do wonders for this role. My #2 pick for Mizuki would be Kugimiya Rie, but if she's picked, Shizuru's voice may need to change, too, because Ryouka/Rie pair doesn't match up well as siblings, in my opinion.
That's all I thought of for now. I can't wait for the DVD release. Since the broadcasting starts in October, I guess it's still months away.
2007-10-08: Well, there went my idea. According to the official site, Shizuru is voiced by Kawasumi Ayako and Mizuki is voiced by Mizuki Nana. I can see Kawasumi-san as Shizuru. I'm not so sure about Nana-chan.
2007-10-28: This is the last update for this editorial. I watched one episode of Mokke anime on youtube. For your information, I was basically set on buying the R2 DVDs, so don't even think about comparing that act to how other people simply download things.
The conclusion is that I'm not even going to touch it. As far as I'm concerned, Mokke anime does not exist. End of discussion.