Drywall circle cutters have been around a long time. But try using one and you'll find they're awkward and clumsy, because they only have one smooth wheel and take both hands to operate. You need one hand to pivot and the other hand to press the wheel through the paper to make the score.
This new circle cutter (from Goldblatt) has two sharp serrated wheels that you pivot around a sharp center tip. The cutter evenly scores the drywall paper surface so you can knock out the plug. It's worth picking one up if you have to cut lots of circles (for recessed lights or round electrical boxes). It is available at most stores that have a good selection of specialty drywall tools.