There's a blog post here
from Tor that talks about the affect (or lack thereof) of ditching DRM.
The title of the thread gives away the ending, but I'll quote a salient section of the post anyway:
We had discussions with our authors before we made the move and we considered very carefully the two key concerns for any publisher when stripping out the DRM from ebooks: copyright protection and territoriality of sales. Protecting our author’s intellectual copyright will always be of a key concern to us and we have very stringent anti-piracy controls in place. But DRM-protected titles are still subject to piracy, and we believe a great majority of readers are just as against piracy as publishers are, understanding that piracy impacts on an author’s ability to earn an income from their creative work. As it is, we’ve seen no discernible increase in piracy on any of our titles, despite them being DRM-free for nearly a year.
I hope more publishers ditch their DRM schemes, and I also hope sellers of other media take notice. I dream of the day I can buy a video file from any service and play it on any device without first stripping the DRM.