I definitely prefer bright text on dark background. Why?
[rant mode]
It's an example of what designers sometimes forget - that screen is much different media than paper. Paper, being white reflects light, giving good contrast. You can usually easily tune the light level (even by tilting the book you read a bit away from the light source) and so it's generally okay. But screen emulating paper is a bad idea. The screen actually emits its own light, and in case of CRT at unhealthy and unpleasant frequencies (both lightwave freq and refresh freq) It's much healthier and much more comfortable to have most of the screen dark and only chosen elements (like text) bright. White as a background may be a nice option for something you view for 2-3 minutes. But if you plan to spend half a hour on the forum, you'll get your eyes deadly tired from looking at so much white.
[/rant mode]

I really miss the monochromatic green monitors. These were pleasant to look at.