RTFM

Using Apple's own PR Library I was able to reconstruct the historical performance of the iTunes music store since it's inception. The following graph depicts the growth curve for iTunes downloads, based on Apples own press releases (and interpolation between dates.)

The pinkish-purple portion of the graph is my conservative estimate of the continued growth of the downloads. (As long as the record industry doesn't pull the plug on Apple.) Based on these estimates, Apple should hit the One Billionth (with a 'B') download in late April 2006.

With the record companies collecting 70 cents for each download, they would have to be completely brain dead to turn off a $500,000,000/year spigot.