Apple's iTunes Store has surpassed Wal-Mart to become the No. 1 music retailer in the U.S., according to new figures from market research firm NPD Group. iTunes surpassed the retail giant based on the amount of music sold during January and February. Launched less than five years ago, the iTunes Store has sold more than four billion songs, and currently features a music catalog of over six million songs. The service pulled ahead of Amazon.com for the No. 3 spot last June, then past Best Buy for the No. 2 spot over the winter. "