A trusted source tells them that the next generation device is actually in the EVT3 stage, the third engineering verification test stage, and has not yet entered the DVT (design verification test) stage.
It's certainly possible Apple is producing engineering samples of new iPhones and has started manufacturing device enclosures to iron out any problems with the manufacturing line before real production starts, but that's all that is happening at this point in time. The final version of Apple's next iPhone - the one that you will buy - has not started production yet.
For a reference on how long each stage takes, the Verizon iPhone 4 reportedly hit the EVT2 stage in mid-November, hit EVT3 the last week of November, and got up to DVT2 in late December.
Additionally, Apple is said to be testing 1GB of RAM, LTE, and NFC for the new iPhone. It's expected to be released at the end of September or early October.