Well, that Amazon MP3 store must not be doing as well as I thought it would. Apple today issued a press release announcing that iTunes is now the #2 music retailer in the United States, trailing only behind retail juggernaut Wal-Mart. Apple also announced that there are over 50 million iTunes customers now, and that on Christmas day there were a whopping 20 million tracks sold in iTunes.

“We’d like to thank the over 50 million music lovers who have helped the iTunes Store reach this incredible milestone,” said Eddy Cue, Apple’s vice president of iTunes. “We continue to add great new features like iTunes Movie Rentals to give our customers even more reason to love iTunes.”

So how long will it be now before iTunes overtakes Wal-Mart? I wouldn’t be surprised to see it in 2008.