Paypal and Square both have a headphone port dongle for accepting credit cards.
https://www.paypal.com/webapps/mpp/credit-card-reader
https://squareup.com/
We already have the hardware demonstrator for accepting magnetic stripe cards http://blog.tehinterweb.com/?p=94 what is needed now is intergration into the API, it seems that Square removed their Android open API but Paypal seems to have a good API if someone wanted to develop an app.

Bitcoin did the first mobile to mobile transfer between two N900's but there is no nice QT app that I know of.