According to MacRumors Apple is already conducting Field Tests for the next gen iPhone in San Francisco Bay Area . The information comes from a company which makes an iPhone application called iBART.
The company Pandav has revealed that its application usage records or if we call it servers logs shows devices with the name iPhone3,1
This is the first time such a device has been spotted in the wild.
We expect the next version of the iPhone to be out in another 6 months