Digg's Kevin Rose is the newest investor in this Square iPhone payment system that we've been seeing lately, and as you can see above, he's demo'ed the unit for everyone over on YouTube. It works as we've heard: there's just an addon that you plug into the iPhone's headphone jack, and then an app takes information from the swiper, and transmits it out for an actual credit card payment, with a finger doing the signature. The app, as Kevin says, will even upload GPS information, so you can make sure that payments are happening in the right place.
Unfortunately, what he doesn't mention is the actual price to get payments up and running. Square's website says that there's "no contracts, monthly fees, or hidden costs," but they've got to pay for the system somehow, so you'd expect there to be a fee for setup, for the accessory, and then a fee per charge through the system, but we haven't heard what any of those will end up being yet.
Depending on how steep they are, this could be a gamechanger -- accepting payments anywhere, any time, with an online record and receipt for each one, no cash involved. Would be incredible for small business owners, and probably a lot of other sales folks, too.