Unless you want your audience to fall asleep, a dark background with light text is a bad idea since you will invariably have to dim the lights or turn them off completely for proper contrast. A light background with dark text (20 pt minimum for bulleted text) is readable and allows one to leave the lights on.
Of course, the brightness of the projector also plays into both scenarios.
Dark Backgrounds
Unless you want your audience to fall asleep, a dark background with light text is a bad idea since you will invariably have to dim the lights or turn them off completely for proper contrast. A light background with dark text (20 pt minimum for bulleted text) is readable and allows one to leave the lights on.
Of course, the brightness of the projector also plays into both scenarios.