Best Buy will become the first independent retailer of the iPhone in the U.S. when it begins selling the iPhone 3G across 970 full-size stores and 16 Best Buy Mobile locations on September 7, reports the Associated Press. “We had a lot of work to do, obviously, to get in a position where Apple and AT&T would feel good about Best Buy Mobile carrying it, and that’s what we’ve done in the last 18 months,” said Shawn Score,…