The new Square credit card reader from Squareup now supports iPhone 4. Square enables mobile merchant transactions on iPhone, iPod touch, and iPad with their free credit card reader (plugs into headphone port on supported models) and their app.
Their new credit card reader is updated for iPhone 4. The main difference of their new adopter is the plastic instead of metal edge near the headphone, following Apple iPhone 4 accessory design notes. It is now shipping.