I just listened to the current
"This Week in Tech" episode 118.
One Guest is Jerry Pournelle, who says:
"I think the next iteration of the iPhone may be the beginning of the end of the paperback book business."
He states, that most people don't buy a dedicated bookreader, but if they are "already carrying something, that is good enough to read a book on" they would consider buying an ebook instead of a paperback. Further he says
"I think the paperback book industry is already in trouble".
There is also mention of the Sony Reader and the Amazon Kindle.
It's worth a listen, this topic starts at about minute 51.
Edit: Another guest, Brad Stone who writes for the New York Times, claims en passant that he has "talked to people who have used" the kindle (59'58).