http://www.un-go.com/
It has been heavily rumored for the last month that BONES, Seiji Mizushima, and Shou Aikawa have been working on a noitaminA show for the Fall season this year. The rumor was that it is a period era style mystery series transplanted into a futuristic setting. The rumor also said that the material would be inspired/based on the detective short story collection Ango Torimonochou, by the late Ango Sakaguchi.
Today the placeholder site for UN-GO went online, with the thumbnail logo embedded in the source code, along with the following:
<meta name="keywords" content="UN-GO,UNGO,アンゴ,アニメ,ノイタミナ,noitamina,公式,OFFICIAL">
<meta name="description" content="2011年10月ノイタミナ枠アニメ「UN-GO」公式サイト。">
<meta name="copyright" content="（C）「UN-GO」製作委員会">
It confirms that the second Fall 2011 noitaminA show is called UN-GO, and it is pronounced Ango. This fits well enough with the rest of the rumor, so I think we can be pretty certain that the general details are accurate. There should be more info very shortly, so I doubt we'll have to wait too long.
Further updated confirmed staff list:
Original Concept - Ango Sakaguchi "Meiji Kaika Ango Torimonocho"
Director - Seiji Mizushima
Story/Script - Shou Aikawa
Original Character Designs - pako, Yun Kouga
Animation Character Design/Supervising Animation Directors - Kazumi Inadome, Yuko Yazaki, Hiroko Yaguchi
Art Design - Takashi Miyamoto, Takeshi Waki
Production Design - Takashi Miyamoto, Junya Ishigaki, Takayuki Yanase
Art Director - Takeshi Waki
Color Setting - Shihoko Nakayama
Director of Photography - Kota Sasaki
Sound Director - Masafumi Mima
Music: Narasaki
Animation Production - BONES
Production: UN-GO Production Committee