For me it appears highly genre-correlated. High percentage of science fiction books come with a small statement "this book is drm free on request of publisher / author". Zero of my photography, music, computer science or graphic novels came with such a tag.
Yeah, Tor Books publishes without DRM, and they seem to be one of the bigger SFF publishers these days. John Scalzi, George R.R. Martin (though not the ASoIaF books), Robert Jordan, Annalee Newitz, Charlie Jane Anders, and a bunch of other SFF authors I recognize. I'm sure there are others, but all the once I've noticed have been from Tor.
Indeed, and I love Tor for this. Brandon Sanderson has also come out against DRM. I already loved the man's books, now I love the man too
same here.
I've also purchased some books that are available as serials on the web for free.
I would imagine those publishers would be aligned with making them .epub