Rachel Donadio in The NYTimes book review section has an essay on the expansion of publishing. As she points out, if reading is generally going down hill this isn't stopping people from adopting self-publishing as a hobby. "Publishing" a book is like putting your photo collection on flickr now-a-days. It is so easy and the hurdles so low that anyone with a half baked idea is doing it. Not that there's anything wrong with that; I happen to believe that the creativity, the experiences and the expression evident in many of these books will become a reference point for our age. Just as letters between family members shone light on a family's history, future generations will browse the published output of family members. I want my parents to do this.
On the other hand, my view may be too prosaic; since many of these self-publishers still think their titles will become the next best seller. As mentioned in the article, the potential for iUniverse.com titles to be on display at B&N garners considerable attention and revenues from their legions of authors. So, there is some delusion but the self-publishing market is estimated to be worth over $1.3billion (although, regretably I can't recall the citation for this number) and deserves significant attention from all segments of the publishing community. As I have suggested before, one of the major publishing houses is going to get into this segment in a big way (Xlibris aside).
As for the number of titles published annually, the absolute number is meaningless without explanation as it grows by 35% from 2006-2007. In order to draw any analysis from this number of published titles the number would have to be broken down considerably. Impressive as the number is it is noted for effect only.