By Kasper Jade & Aidan Malley

Published: 07:00 PM EST

Best Buy Co., Inc. is expected to announce as early as Wednesday 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 Cupertino-based 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.  Before, at least some of these phone sales would have been handled by more superficially knowledgeable floor workers.

The program is already credited by Best Buy as having sparked interest in premium, media-savvy devices like the Samsung Instinct, which is one of iPhone 3G's most direct rivals and is only available through Best Buy and Sprint.  Sales of this class of device have grown tenfold year over year at each store that has a Best Buy Mobile presence, according to the retailer.

Appropriately, Carphone Warehouse -- which itself is the exclusive third-party reseller for iPhones in the UK -- has been coordinating the rollout of Best Buy Mobile ever since the effort began in 2006.

Best Buy is known to have been angling for its share of iPhone sales a full year ago with the initial goal of carrying the touchscreen device itself by that fall, capitalizing on a surge in holiday demand. Those people knowledgeable of the earlier plans now tell AppleInsider that the store chain was set aside by Apple after rampant unlocking became a concern, even with purchases from Apple's own stores.

The rejuvenated expansion plans could be essential for Apple, which has seen long lineups at U.S. stores ever since iPhone 3G first went on sale. Best Buy's existence as a third source will expand the number of distribution points just as the holiday shopping season begins and demand increases, potentially giving customers more places to buy the phone while dodging lengthy queues that would otherwise get worse without more points of sale.

Representatives for Best Buy did not return calls requesting comment by press time.