Currently, the enterprise-level Adobe LiveCycle ES product allows users to embed advanced DRM restrictions to their PDFs - printing, copying, and time limits are all supported. A major component of this is a 'policy server' which can be set up locally or by Adobe to monitor the ongoing usage of DRM-enabled PDFs. The end user is notified via a dialogue box that the author of the file is monitoring its usage.

Given the explosive growth of personal publishing via the web (ebooks, blogging, etc), I think there's a great case for making these capabilities available to non-enterprise content authors. You could easily set up a web services situation that would allow a blogger to pay a monthly fee for the ability to monitor and better restrict his/her ebook usage.

Another example: a small business publishes a short guide/ebook that they wish to license out to other organizations. For a small monthly fee, the licensee can print as many copies as they want of the ebook. By allowing that licensor access to this particular web service, you could facilitate their management of their various licensee's, and allow them the ability to protect their intellectual property by monitoring.

Creating a product like this would open doors for the millions of users who need this sort of functionality, but for whom Enterprise-level software is cost-prohibitive.