I'm pretty sure all of us season 1 and 2 viewers saw this one coming eventually. Although it's not a formal announcement, this was leaked in the beginning of October that I kept forgetting to bring around. It was hinted at the Dengeki Book Autumn Festival where Rie Kugimiya (Shana) and Satoshi Hino (Sakai Yuuji), the novel author and illustrator "accidentally" hinted that the third season was possibly in the works basing on the last parts of the novel (see source below)
It's not a big deal that I forgot though since it doesn't look like it will be starting anytime soon. My bets are on Fall 2009 at the earliest at this rate, especially since there was a 2 year gap in between the first and second season. It would only be appropriate if they gave another 2 year gap to give the novels some time....though I'm hoping for a much thorough and meaningful plot this time unlike season 2.
Episodes: 24 (most likely just like season 1 and 2, if not 12 or 13 due to little content left to cover from what I know)
Premier: Fall 2009? (unconfirmed)
Source: http://www.sankakucomplex.com/2008/10/05/n...o-shana-season/
The link to the Japanese Dengeki news article is also within the source if you are able to read Japanese that is....