Summary of the Annoying PowerPoint Survey. "Lose the black background (and the white one) and talk to a designer." [ComputerZen.com - Scott Hanselman's Weblog]
It seems like most of the decks I see used internally (and up the management hierarchy) are "plain" / default design.pot [black text + white background]. When things start to go outside of the team (or public), then things start to look a little bit more colorful.
Personally, I like "plain": if it's a choice between me finding bugs and me spending time experimenting with color schemes/formatting, I lean towards finding bugs. Of course, my design skills are spartan at best.