Non-DRM ebook stores list, in addition to
Three Press Consulting's list of DRM-Free Ebook Publishers, some of whom don't have specific ebook stores.
Three Press's list has 8 nonfic, 19 "regular" fic, and 40 romance/erotica publishers listed.
(Romance/erotica has my vote for "genre that will break the DRM hold on the ebook industry," because there is lots of competition for the mainstream publishers. Harlequin's doing very well with DRM'd ebooks, but their lack of range means dozens of small imprints are also making a living.)