The cast list is out, and it stars the seiyuu I am most enjoying right now, Yuuki Aoi. As one post on 2channel said, it's hard to imagine her in a "boing" (i.e., big-breasted) role. Her range of characters just keeps growing, from Murasaki to Riko to Ichigo to Mina to Korone to Yagyuu Juubei.
Both of the stars -- Yuuki Aoi and Kotobuki Minako -- are 18. There is going to be a radio show, and those two are good bets to be on it, since they both bring something special to their radio performances: in Ao-chan's case, verbal agility and sharpness; in Mina-chan's case, bizarre and entertaining outbursts.
柳生十兵衛：悠木碧 Yagyuu Juubei -
Yuuki Aoi
徳川千(千姫)：寿美菜子 Tokugawa Sen (Sen Hime) -
Kotobuki Minako
真田幸村：釘宮理恵 Sanada Yukimura -
Kugimiya Rie
柳生宗朗：平川大輔 Yagyuu Muneakira -
Hirakawa Daisuke
後藤又兵衛：小林ゆう Gotou Matabei -
Kobayashi Yuu
シャルル・ド・ダルタニアン：小清水亜美 Charles de D'artagnan -
Koshimizu Ami
柳生義仙：水原薫 Yagyuu Gisen (?) -
Mizuhara Kaoru
直江兼続：豊崎愛生 Naoe (?) Kanetsugu -
Toyosaki Aki
服部半蔵：後藤沙緒里 Hattori Hanzou -
Gotou Saori
徳川慶彦：櫻井孝宏 Tokugawa Yoshihiko -
Sakurai Takahiro
Terrific cast. Here is a poor but large-scale image of a couple of magazine pages with pics of the characters and their seiyuus. Click to enlarge:
All the sources I see just say "2010" for the broadcast date, so I guess October is likely.