The latest idea from Jack the inventor of Twitter is called Square. Its a dongle that is attached to the audio jack of your iPhone that allows you to swipe credit cards and make a secure payment on the iPhone. Its like PayPal in the way there are contracts or monthly fees just a percentage is taken from the transaction.
The creditcard owner gets a receipt sent to their email address or mobile phone straight away and cabn access them securely online at the squareup.com website. You can SMS to authorise every payment in real-time.
In addition to swiping your credit card using the Square dongle, authorisation of the card is also verified by using your finger to electroinally sign the screen of the iPhone as shown in the image below.
The YouTube clip below demonstrates the Square in a beta mode being demonstrated at a tradeshow.