As per Yu Aida (creator of Gunsligner Girl), The second season starts airing in Japan on 12/22!
 
It follows the series from Manga (Magazine print) Story from Volume 3 and ends in Volume 6 in another 13 Episodes.
 
When the DVD is released in the US in Summer '08, it will have 2 extra episodes, making 15 total.
 
This is the link to Yu Aida's Website, about GsG (translated from Japanese by google):
http://translate.google.com/translate?hl=en&sl=ja&u=http://www.remus.dti .ne.jp/~jewelbox/&sa=X&oi=translate&resnum=1&ct=result&prev=/search%3Fq%3Dhttp://www.remus.dti.ne.jp/~jewelbox/%26hl%3Den%26sa%3DG
 
 
For those interested, I posted the mangas online for you to compare and see the 'future' of GsG.
Vol. 1
Vol. 2
Vol. 3
Vol. 4
Vol. 5
Vol. 6