A copy of an internal Apple email to employees indicates that iTunes has surpassed Wal-Mart as the #1 music retailer.  Or has it?The news was announced in an e-mail sent this afternoon to some Apple employees, a copy of which was seen by Ars Technica. It includes a screenshot of an Excel file showing the top ten music retailers in the US for January 2008, and Apple is at the top of the list. The iTunes Store leads the pack with 19 percent, Wal-Mart (which includes the brick-and-mortar stores as well as