As the Apple Turns covers the list of purchasable songs of 'silence' available for single song purchase on the iTunes music store. At this moment, John Cage's famous 4'33" is not one of them.
Curiously, this morning I was doing some final edits on a new Eucci track for an online label to check out, and somehow managed to zero out all the audio - producing nearly 14 minutes of silence. It's not as much as this guy offers [iTunes Link], but I did make ask if he was interested in it.
(The original piece is in fine shape at the studio)