The iPad already has an app for credit card transactions thanks to Twitter creator Jack Dorsey’s Square project.

Originally planned for the iPhone, the app works with a peripheral credit card scanner that attaches to the headphone jack. Once the scanner is in place, you can use your iPad as a mobile cash register.

Customers will be impressed by the multi-touch interface, and they will be able to sign for their purchases on the screen. The app then verifies the credit card transactions through Square’s servers for a flat fee plus a small percentage of the sale.

In some cases, this application will be easier than using a regular credit card machine. An art dealer, for instance, could approach a potential buyer at a gallery opening with the iPad in hand to make a sale.

After the sale is completed, you can send receipts to customers via e-mail or SMS. Here are some pics.