The end result is not intended to be about "silence" because when you listen closely you can hear signs of life all around through the normal noise that would occur in any room ( even the sound of traffic outside if you listen closely apparently), which is the point of it really. Who said only instruments can be used to perform a composition?
Of course in the context of it being a free ITunes download I think it is even more brilliant. I am sure that those hunting for a free Lady Gaga track, or some other piece of musical ephemera may have found this a little baffling. It is wonderfully playful of ITunes to do this, and I like how it subverts the usual formula of providing some hook laden piece of pop designed to provide immediate gratification. Cage's piece actually forces you to listen.
In the spirit of this I have a few other suggestions for ITunes to consider;
"The Best of Marcel Marceau" (audio podcast). - An obvious one I admit, but I am somewhat intrigued around how one would visualize the movements of a graceful mime based on the natural rhythms and sounds created by his movement?
Lou Reed "Metal Machine Music" (locked groove edition) On the original vinyl LP the last 1.8 seconds of the album was repeated on an endless loop on the "locked groove" (see wikipedia entry for a decent explanation of the "locked groove" phenomena with vinyl LPs) Unfortunately an MP3 file cannot go on ad infinitum without taking up all the precious space on your hard drive, so I am sure that a decent five minute sample of the locked groove played on repeat would have a similar effect. Infinity here we come.
"The Best of Category Sixx" - Apparently Category Sixx are a "well known" air band that play the bar circuit. Of course due to copyright reasons the original songs that they play along to would be omitted and we would be left with the sounds of the live audience, as well as whatever sounds the band manages to produce jumping around on stage with their imaginary instruments. This would be strangely captivating I imagine.
"One Hand Clapping - The Hits" - the current top 10 downloaded singles as expressed in real time through one handed clapping. Are you astute enough to pick up what song is being expressed through the movement of air in an empty space?
Of course I would love to hear other ideas for an unconventional download.