Best Buy is expected to announce plans to begin selling the iPhone 3G next month, making it the first independent U.S. reseller to offer the the coveted touchscreen handset outside of Apple and AT&T.

People familiar with the matter say the electronics retailer has reached an agreement with Apple that will see the iPhone sold at 970 Best Buy stores in addition to 16 Best Buy Mobile specialty outlets beginning September 7th.

Best Buy already offers Apple’s complete line of iPod digital music players at all its stores and recently expanded sales of the company’s Mac computer line to more than 600 of its larger footprint locations.

Word of the move comes just one week after Best Buy announced that it has completed upgrading its Best Buy Mobile departments inside each of the company’s U.S.-based stores — a factor which may have been instrumental to the iPhone deal.

Every facility now has a dedicated cellphone section with similarly committed staff who are trained 80 hours each to help customers buy mobile devices. The focus and reach of the program may have swayed Apple, according to people with knowledge of the situation.

The design of the iPhone plus the 3G upgrade continues mushrooming sales for Apple. Criticism of AT&T’s 3G network and the possibility of a recall based on potentially flawed Infineon chips don’t seem to have slowed sales at all.