iPhone accessory maker Mophie has something interesting up its sleeve. They will be offering a credit card reader for the smartphone along with a transaction app. The whole thing is integrated into an iPhone case, so you can keep the reader on permanently, as long as you don’t mind the added bulk.
As long as the app is good and the price of the service is affordable, this should do well. Assuming you use your credit card often anyway. We should get more info at CES 2010.