What kind of frosting doesn't need to be refrigerated?
I've never had a buttercream frosting consisting of nothing but butter, powdered sugar and vanilla (and often cocoa or coloring) go bad. Even when it's got a little milk in it. There are two reasons for this. One, it doesn't last long enough. Hey! Where'd that last piece of cake go? Two, though I'm no expert, I think it is the high amount of sugar as Michael alluded to. I've had the infrequent cake with a piece or two that have hung around for a week without any problems other than a little drying.