Looks like there's something to this "downloading music" thing after all -- just eight months after topping Amazon, iTunes has now passed Best Buy to become the number two music retailer in the US. Apple says 50 million customers have bought over 4 billion songs, with 20 million sold on Christmas day alone. That leaves just Wal-Mart in first place, but with the percentage of digital music sales growing each quarter, ol' Wally may not be safe for long. ...