A company, DeviceFidelity, is currently working in conjunction with Visa to incorporate Visa payWave into the case for an iPhone.
The Visa payWave product has been around for a number of years using a traditional card bearing the Visa logo. But the new iPhone cases will have a chip built into them to make them work the same way.
The reason why the credit card companies want you to use the PayPass or payWave products is they found out people are more likely to use their card more often if it is less hassle to process the transaction. More use leads to more fees for the banks and Visa. That makes them happy.