According to this ClickZ article MP3 Player Adoption at Tipping Point, there are now 22 million Americans with iPod’s or other MP3 players and this market has reached a “tipping point” that is making it viable as a distribution platform. I’ll sure be amazed if podcasting takes off. Probably we just need more people with something to say, or who REALLY want to be HEARD. 😉
Of course, it always takes a while for folks to learn to get their information and media in new ways. Witness the slow ramp-up of Tivo and PVR’s.