I agree Christensen often seems a bit post hoc, but I think at least trying to see if there's a new market can help (as opposed to attacking the fortified hill of an established market).
One could argue the Nomad's market was much smaller (most people who bought iPods had never heard of it), partly because of Apple's aggressive advertising... which certainly helps "create" a market.
In the case of the iPod, the answer is surely the iTunes Music Store, which was the first legal online store that sold popular music in digital formats.
