Popcorn is about 3 net carbs per cup, popped, so as cheats go it's not the worst thing in the world. Certainly better than popping a giant box of Jr. Mints or a king-size Snickers. Like potatofree said, though, it is pretty high on the GI scale, which is why it's not considered an Atkins-friendly food.

That said, I have been known to grab a handful from a friend at the movies. Be careful, though, as 1 cup of popped popcorn is NOT much at all. You could easily eat 10 cups before you realized it was gone.