Check out Cube. I have installed the app, but haven't bought the reader yet. If your phone is not on the confirmed device list, you can request a free reader, if you agree to be a tester for your model phone.
I don't know when I'll buy it. I haven't had any customers request to pay via a card. If they do, I have a reader from Intuit that was provided for free and an iPod touch to process the funds.