browsee - 7:00 pm on Apr 14, 2011 (gmt 0)
I am not saying that background is the only factor. I am saying that background is one of the important factors. There are 200+ signals in Google algo(old algo). Age of Site, Backlink ratio etc are the important signals in the Google algo. Like wise, thin pages, ads above the fold, bg colors are important factors in the new algo.
Sites you mentioned in your post cater to young audience right. Young people prefer dark colors. Probably algo is using it as one of signals.