Also, are all DRM services device-specific? Is there some kind of common, open DRM protocol? (I am aware that your answer, d4nj450n, seems to indicate that there is no open protocol).
It's more the other way around, devices and apps support a given set of DRM schemes and file formats. Notably the Kindle does not support the EPUB format at all. Open DRM systems do not really exist, mostly because DRM tends to rely upon secret algorithms that once reverse engineered make it trivial to defeat the protection (unlike, say, normal encryption where security through obscurity is not necessary).
It's more the other way around, devices and apps support a given set of DRM schemes and file formats. Notably the Kindle does not support the EPUB format at all. Open DRM systems do not really exist, mostly because DRM tends to rely upon secret algorithms that once reverse engineered make it trivial to defeat the protection (unlike, say, normal encryption where security through obscurity is not necessary).