Is this true? I think Amazon has more titles then either Nook or Sony. Sony is very "restrictive" in who can be on there - Amazon is VERY open now - but I agree if they start moving away from that that will not be good for them.
I know that robin has left us now, but I thought I would clarify this. I'm talking about the devices themselves, not whether or not it's easy for a self-publishing author to sell through the Sony store. (although with PubIt, selling through the nookstore is now as easy as Amazon.)
The Sony bookstore has a smaller number of books available than Amazon. The nook bookstore has more than twice as many (2 million as opposed to about 730,000 as of the release of the nook color last Christmas season). However, the key to the Sony and the nook are that they read a format that is sold not only by their respective stores. The Kindle still doesn't read ePub, because Amazon wants to tie its users to their content. So any writer wishing to publish for Kindle either has to bootleg their book, or sell through Amazon directly. Kindle users have 730,000 books to choose from (in the US), and maybe some extras from bootlegging.
Sony and nook, on the other hand, have almost limitless reach. Books that work for the nook and Sony (and iPad) can be bought legally from huge numbers of sources and do not have to be bought directly from the store associated with that reader, the biggest non-device-related store now being the Google e-book store. In the long run, I think this will give those devices a competitive edge if they can win out over Amazon's early lead, and I think authors will be rightfully skeptical of publishing with AMZ in a way that limits their legal buying market only to people who own Kindles.