A reminder that DRM is for text too

eWeek:

Adobe Systems Inc. on Monday acquired the digital rights management business of Navisware.  The purchase, for an undisclosed sum, allows Adobe to jump into the burgeoning DRM sector and deploy its own technology for protecting Microsoft Office documents. The FileLine DRM products from Navisware are designed to provide security and version control for CAD, Office and other file types.

Question that needs answering (brought up during my visit to the Berkman Center at the Harvard School of Law yesterday): If all these DRM systems are incompatible with each other, what happens when DRM gets applied to electronic books (one day, maybe the only form of books)?