Cory's latest Guardian column is about "Why free ebooks should be part of the plot for writers," but he also addresses the question of self-publishing:
[W]hy do we need publishers if we can just release ebooks and make the print available through one of the many excellent print-on-demand houses such as lulu.com? Well, a traditional publisher does a lot for you that is unrelated to printing books, from preparing the manuscript to ensuring that the book connects with an audience by wooing reviewers to talk the book up, booksellers to put it in the path of readers, librarians to put it on the shelf and, of course, by paying for a certain amount of marketing in the speciality and general press.
Note: that's not a typo! People really do say "speciality" instead of "specialty" in England!