The Distributed Computing Industry published a white paper, "Digital Watermarking Technologies - Applications in p2p networks." The paper outlines how digital watermarks can be used in P2P networks to provide benefits to all constituents: consumers, P2P system operators, Internet service providers (ISPs), and content owners.

Digital watermarks are digital data elements that are embedded into, and read from, video, audio and still images. They allow digital content to be self-identifying or carry information that may trigger a defined behavior. The digital watermark data is not perceptible to the human eye or ear, but can be read by computers.

Watermarking is supported by the MPAA. It's a strong non-obtrusive way to track content. Unfortunately it's been slow to take off.