Nero has announced that they will be releasing a new version of Nero 7 in April that will add a copy protection feature to their SecurDisc technology. Here's part of their press release:
Nero, leaders in digital media technology, announced today that the April update of the Nero 7 digital multimedia suite will include a new feature of SecurDisc technology. Now users can benefit from an innovative copy protection feature designed to prevent duplication of PDF documents saved on DVDs created with SecurDisc.

SecurDisc is a technology that offers a full feature set of security-related functions through a combined hardware/software solution and additionally offers encryption with password protection, digital signature, and higher data reliability through data storage redundancy and defect management.
From what we've seen, SecurDisc works quite well. Unfortunately, its only supported by a few drives at the moment. If you'd like to read more, Nero's entire press release can be found here.