SquareUp looks like it could brighten up the currently dismal selection of credit card processing options for very small business.
The part about "Read payment cards from any device with an audio input jack, including your mobile phone" sounds pretty interesting to me.
The key feature though, is probably this: "Start accepting payment cards immediately with Square. No contracts, monthly fees, or hidden costs."
I'm going to keep an eye on these guys and see how they do.