Apple are preparing to sell their 500 millionth song, which is a lot of downloads sold. As with the previous landmark sales, the iTunes Music Store will be running a promotion around every 100,000th song sold in the run up to the half-billionth sale (I think this is in the US only, though it's hard to tell, and Apple's PR people aren't the most helpful in the world).
A counter will keep track of how many tracks have been sold. The purchaser of each 100,000th track will get a 50 song gift certificate, while the music fan who buys the 500 millionth track will get ten iPods to share with their friends, a 10,000 song gift certificate, and an all-expenses paid trip to see Coldplay - which is enough to make you want to buy the 500,000,001th track.