The mystery of Steve Jobs’ iPhone retail strategy deepens with reports that Apple (AAPL) will begin selling its smartphones at Wal-Mart (WMT) three days after Christmas.

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:

In a move that will significantly expand its retail presence in time for the holiday season, Apple has agreed to let retailing giant Best Buy sell the new iPhone 3G through its nationwide chain of Best Buy Mobile outlets starting early next month.

Best Buy markets cell phones in the United States through 970 full-size stores and 16 stand-alone Best Buy Mobile shops. All U.S. Best Buy stores will carry the iPhone except for a handful of outlets located in areas where AT&T does not provide cell phone coverage.

The deal, first reported on Tuesday by Apple Insider and confirmed by Best Buy Mobile president Shawn Score (see here), could serve both companies well.

For Apple (AAPL), which has been struggling to meet the extraordinary demand for its second-generation iPhone through its smaller network of Apple and AT&T retail stores, the deal puts its hottest-selling product in the hands of one of world’s savviest retailers. Best Buy, a Fortune 100 company, is the world’s largest consumer electronics retailer, with a 21% share of the U.S. electronics market and a 3.6% share of the cell phone market, up from 2% last year.

For Best Buy (BBY), which has been angling for the iPhone business for more than a year, the deal will add Apple’s cachet to its expanding smartphone offerings and help drive traffic to new Best Buy Mobile departments within its stores. Best Buy is aggressively marketing a variety of smartphones, from RIM (RIMM) BlackBerry Curves to Palm (PALM) Treos, and is the exclusive reseller, with Sprint (S), of the Samsung Instinct, one of the iPhone’s nearest competitors.

Apple and Best Buy have been slowly expanding their relationship since the retailer began carrying iPods in 2002. Best Buy started selling Macs in selected stores in 2006, and recently expanded the program to more than 600 outlets.

The deal can be seen as a victory for Best Buy’s “consumer centricity” marketing strategy, by which it caters to the needs of specific types of customers in specialty boutiques within its full-size stores. Last week Best Buy announced that it had completed a nationwide roll out of its Best Buy Mobile store-within-stores, a joint venture with Britain’s CarPhone Warehouse that began in 2006 and has led, according to Best Buy, to a 10-fold increase, year-over-year, in high-end multimedia phone purchases (link).

Best Buy, based in Richfield, Minnesota, operates more than 1,150 stores in the United States, Puerto Rico, Canada, China, Mexico and Turkey. Earlier this year it purchased a half-share of CarPhone Warehouse, which has 2,400 outlets in nine European countries.

Apple operates 219 stores, 187 of them in the United States, where customers have been queuing up for the iPhone 3G since early July. AT&T (T) sells iPhones in some 2,000 stores, but the current waiting period for customers who want to buy one from AT&T is 7 to 10 days.