I believe in using a black background for talk slides.

Example: http://new.livestream.com/gigaom/gigaomroadmap/videos/579403...

I would prefer dark gray from an aesthetic perspective. But the reason I choose black is simple:

Often, the LCD projector is not pointed straight at the backdrop or maximized correctly. If your slide background is not black, there is a perceptible margin, and your slide area is shaped like a trapezoid, not a rectangle. Using a black slide background minimizes this margin, and makes it look like your slides correctly fill the backdrop.

Just my two cents. I'm curious if people feel strongly against this approach.