According to the Boy Genius Report, a blog with relatively reliable backchannel sources at AT&T, Apple will start with select Wal-Mart and Sam’s Club outlets on Dec. 28, and eventually roll the iPhone out to 2,500 Wal-Mart owned stores.

The deal, if confirmed, would represent the fourth major expansion of the iPhone’s retail presence outside Apple’s own 200-plus stores — first to AT&T’s (T) 2,000 retail outlets, then to nearly 1,000 Best Buy (BBY) outlets (see here), and then to the tens of thousands of points of sale (many of them no more than mom-and-pop kiosks) that carry iPhones for Apple’s overseas partners.

This is a move of different magnitude. Wal-Mart is the world’s largest retail chain — by far — with more than 7,000 mega-stores around the world and some 2.1 million employees. It finished its last fiscal year with nearly $380 billion in sales — earning it the No. 1 slot in the Fortune 500.

All of which raises the question of timing. If you’re going to bring the iPhone to Wal-Mart, why not do it before Christmas — as originally rumored — rather than after?

Below the fold: An AT&T memo with a timeline of the Wal-Mart rollout, as posted Wednesday by the Boy Genius Report:

We are pleased to announce that Wal-Mart has reached agreement with Apple to offer iPhone 3G in Wal-Mart and Wal-Mart-managed Sam’s Club* Connection Centers nationwide beginning December 28, 2008.  AT&T will support Wal-Mart and Sam’s Club with the activation process for iPhone 3G, and we need all National Retail employees engaged to ensure a successful launch.

*NOTE: This product launch DOES NOT apply to SC Kiosks within Sam’s Club managed by Radio Shack.

Wal-Mart has communicated the following launch plans and execution timeline to their employees:

• iPhone 3G will be available in-store only beginning 12/28/08
• New and existing customers in good standing can purchase iPhone 3G with a 2 year contract
• iPhone 3G transactions only will be processed through POS.com, not the kiosk

Execution Timeline:

• 12/2 – 12/5:  AT&T / Wal-Mart MEM Classroom Training for Store Leads
• 12/3 – 12/19:  AT&T In-Store Training
• 12/26 – 12/27:  AT&T In-Store Refresher Training
• 12/28: Nationwide Launch
• 12/28 – 12/30:  Wal-Mart and AT&T War Room Support

(link)