Apple’s iPhone 3G and iPhone 3GS continue to top the charts for wireless partner AT&T, according to a new research report from Piper Jaffray. Based on checks of the channel by Piper Jaffray’s wireless analyst, Mike Walkley, the iPhone is not only AT&T’s top selling device, it is also taking share away from its competitors.

“Overall, the iPhone remains the best selling device and our August checks indicate the iPhone took share from BlackBerry, the Nokia E71x, and most other competitor products,” said Piper Jaffray Senior Analyst, Gene Munster, the author of the report.

The news only gets better for Apple. Munster also checked Best Buy and store managers indicated that the iPhone was their best selling device as well. Best Buy said the stores were selling out of stock as soon as they received iPhones from Apple.

Munster believes that Apple will sell 7 million iPhones in the September quarter.