For thorough reading, I much prefer to have dark text on a light background. However, I can also easily read light colored text on a dark background. The following is an example of a site I find highly readable color wise: http://daringfireball.net/. Another example of a site I find highly readable but has the exact reverse combination is this one: http://www.456bereastreet.com/
Both sites do not use black and white, but slightly lighter shades. I prefer lighter shadings because my eyes are sensitive to light, ergo a hard black/white or white/black approach makes my eyes sore and strained in a matter of minutes. Unfortunately, many "purists" who claim that anything that isn't #000 or #fff is mere design idiocy do not account for people like me who suffer from light sensitivity.
With that said, I've not found a single source that has some hard facts regarding the matter without counter articles stating the opposite. There's been a thread here on SP some time ago where a couple of people with a form of dyslexia said they preferred light text on dark backgrounds, though far more people who have a case of dyslexia have claimed the exact opposite. An absolute answer to that question seems to be non-existent. The only conclusion I draw is that high contrast is mandatory regardless of the approach you use in the decision-making process.