A few minutes fighting with google I found these:
http://blog.tatham.oddie.com.au/2008...is-a-bad-idea/
Cites a 1980 study where people seemed to improve their comprehension 26% with dark text on light background.
This one suggests the opposite that black backgrounds is better for night time driving GPS and dashboards displays:
http://www.perceptualedge.com/blog/?p=1445