Wal-Mart will begin offering the iPhone 3G at select Wal-Mart and Sam’s Club locations on December 28, according to a supposed internal company memo posted online. The Boy Genius Report, which previously suggested the retailer would begin offering Apple’s handset on November 15, has posted the alleged leak, which states: “We are pleased to announce that Wal-Mart has reached agreement with Apple to offer iPhone 3G in Wal-Mart and…