I read with interest today that Amazon says 148 ebooks sold for every one hardback this year and in the last month it was 180 ebooks for every hardback. Raises the question of whether or not print books are over. I don't see it that way but certainly ebooks will become a large part of the total book market.
Its estimated that this year ebooks will be a 500 million dollar market, the total book market is about 23.5 billion. So ebooks are becoming a significant part of the book market but still in its infancy and will be for some time.
I think the tipping point will be when an ipad like erearder will sell for less that $200.00. It appears that the ebook market has exploded in the past year but after we get offering of a sub 200.00 dollar ipad it will probably take off like a rocket ship.