I believe it is a 'push me/pull me' scenario between how many books are available on Kindle and how many Kindles are sold. The more readers, the more books will be offered. The more books offered, the easier it is to convince readers to buy a Kindle.
At first there were 90,000 books on Kindle and a mere 8,300 more per month were being added. Then the slope changed after June 2008. The slope almost doubled to 14,800 books per month. In 2010 there is another 'step change.' The rate books have been added to the Kindle more than doubled the 'mid' rate to 35,450 books per month!
The publishers should worry. I suspect the 14,800 books per month was the quantity of books the publishers wanted released to Kindle. The quantity of books being offered looks to be ready to prove the 'long tail' theory.
The variety of books being offered will pull readers to ereaders. By mid-2011 we should expect to have over a million books for sale on Kindle (plus the 1.8 million+ public domain books already available). Anyone who doubts the growth of ebook market share needs to ponder the impact of having such a quantity of books less than a minute away...
PS (late edit)
We should see another doubling in mid-2011. How? I expect it to be amateur authors putting their novels out. Or maybe it won't be books but rather an effective doubling of offered text via Amazon's 'singles.' I'm not a writer so I can be the 3rd party observer just wondering what more will be offered for my enjoyment.