Over on his blog author "netxing" has uploaded a post describing how he was able to use a Portapack to spoof mag-stripe info stored on credit/debit cards. The idea based around an old trick called magnetic stripe audio spoofing. This is essentially using an electromagnet and a music player like an iPod or smartphone to trick a magnetic card reader into thinking that you're swiping a card through it.
Netxing's idea was to use an FM transmitter connected to a computer to transmit known magnetic stripe card data via FM to the Portapack. The Portapack then receives and outputs this as FM audio to an electromagnet connected to the audio out jack, allowing it to activate the magnetic card reader.
Using this method it could be possible to make a payment by transmitting card data remotely over an FM signal. We're not sure on why you'd want to do this, but it is an interesting experiment regardless.