Continuing its dominance in the music industry, iTunes has passed yet another milestone in its storied history.

With the number of iPods and iPhones increasing ever more rapidly, sales on iTunes have followed a similar path, and have also been accelerating as well.

Apple sold its 1 billionth song in February 2006, a little less than three years after its April 2003 launch. It would be less than a year until it sold its second billion -- January 2007 -- and about six months each for its third and fourth.

Earlier this year, iTunes surpassed Wal-Mart, making it the US' largest music retailer. The rise has been nothing short of meteoric: As recently as the third-quarter of 2005, it was ranked seventh among retailers including Circuit City, FYE, Amazon.com, Target, Best Buy, and Wal-Mart.

Several competitors have attempted to thwart the upward rise of iTunes, including Amazon. While it thought offering MP3s may have been the key, NPD data shows that only 10% of its userbase is comprised of former iTunes customers.

What's worse is even though iTunes doesn't use MP3 for its protected files (opting for AAC) and Amazon has a much larger catalog of unprotected tracks, the Cupertino company still manages to sell ten times the amount of DRM-free music as Amazon.

iTunes' success in music seems to now also be transferring to its movie rental and sales business. Apple reports that 50,000 movies are either rented or bought each day, making it the most popular online movie store.

The current catalog of iTunes includes some eight million music tracks, 20,000 television episodes, and 2,000 films.