I think Koland has a point.
The comparison is of course meaningless and stupid but if you do compare the sales figures of the ipod to the Kindle estimates, they are in most respects equivalent.
http://www.systemshootouts.org/ipod_sales.html
One important point is that when Apple introduced their player in 2001, most people already knew what a mp3 player was all about, so the marked was already there, but most people does still not seen an e-reader live.