I've been advocating for this for a looong time dept. DRM level 2
Watermarking to Replace DRM?
"News.com has an article on the announcement of Microsoft and Universal to introduce watermarking technology into audio files. The technology could serve several purposes including tracking file sharing statistics and inserting advertisements into audio tracks. The article goes on to suggest that watermarking could possibly replace DRM in the near future."

this should be applied to ebook and emag too.