This was actually rumored way, way back in July 2008, when a document with info on BONES staff as well as word of a new Darker than Black AND a new Fullmetal Alchemist were in the works. Obviously the FMA news was later confirmed, but even if it had been true at the time, that's never a guarantee that a project will reach fruition. However, DtBII is now far enough along that they're willing to tell us about it, which is groovy.
That said, there's nothing else to know about it, really. The image features Hei and Yin from the original series, but if it's just a promo image that doesn't even necessarily prove that they'll appear or be the main characters again-- we could wind up meeting new Contractors dealing with what happened at the end of the series.
...Or they could just bring us back the very popular characters. Hee.
I'm really excited about this one, guys, because Darker than Black is a fantastic, solid show. It's not the kind of show that becomes your primary sub-fandom, it's not The One True Show, it won't change your life. But it's got a solid world setting, interesting characters with excellent designs, a cool ongoing story combined with random vignettes and other interesting side stories, and, oh yes, fantastic music-- Yoko Kanno did the soundtrack.
If you haven't already checked out FUNimation's release of the show, I highly recommend doing so; you can snatch it up pretty cheap for $15-20 used on Amazon.com or new for $22 on Right Stuf. If you like a solid supernatural action/mystery (and who doesn't?) it's worth your time to give it a shot.
This has made my day!