We don’t know many details, as Sigma likes to keep us waiting in excitement for September 26 to come - that’s another month!
Sigma reveals a few details in a flash movie. They mention:

• an extended color range
• pixels that catch more than one color
• improved raw images

Sigma has always been honest about pixels: 10.29 Mp (3.4Mp Red + 3.4Mp Green + 3.4Mp Blue) (from the SD10 specifications) so we’re curious how the Foveon chip in this latest DSLR will process color information!