I can define it from two directions: the first, starting with 'social,' woudl do for design what digg does for 'social news' or del.icio.us' does for 'social bookmarking.' Social design would mean design collaboration on a massive scale, generated by ordinary people, who acting in concert are smarter than any one individual.

I could also see 'social design' ass being defined from the other direction, starting from an individual designer using his or her skill to express a social conscience, making an impact on society through the pwoer of graphics and text.

Either way, I think what Design21 is doing is incredibly cool, and I'm happy that they've partnered with GOOD.