My university tried this and got sued for it. These things are not yet ready for prime time because they are not accessible enough for those with disabilities.
here is the results of the court case: http://www.ada.gov/case_western_univ.htm
here is the results of the court case: http://www.ada.gov/case_western_univ.htm
However, a general interface to a Braille display is necessarily one which transmits the letters of the book in reading order, so I don't see how they can implement that without opening up a big gap in their DRM.