DRM for Gnutella?
 

Digital restrictions management for an open-source peer-to-peer network. ( http://www.cc.gatech.edu/~judge )Researchers at the ( http://www.gtisc.gatech.edu ) Georgia Tech Information Security Center have created a content protection system that is a plug-in for LimeWire/Gnutella. The (http://www.cc.gatech.edu/~judge/research/position.pdf ) paper argues that DRM is beneficial to everyone including independent musicians and end-users."

Maybe you would like to implement "OpenMG X" ? ;)

Sony Corporation today announced "OpenMG X", a digital rights management and distribution technology which is utilized for various types of products and devices. This technology will play a key role as the market for music and movie content downloaded via the Internet expands.
"OpenMG X" flexibly adapts to the distribution of content to PCs, as well as services which distribute content directly to AV and mobile devices. With this technology, the usage conditions for content can be controlled from the distributor's end and hence, content distribution can be secured from the beginning to the end of the service. This technology will be promoted widely to music labels and other music/content distribution companies to use as a core technology for protecting their content

Link to OpenMG X's announcement.