Google has filed a patent for what it calls user-distributed advertisements (UDA). A UDA system "facilitates insertion of manually selected ads into a document that is to be distributed (e.g., transmitted, published, and/or posted) such that the document is to be made available to other users. For example, manually selected ads can be inserted into an email to be sent to another user, a blog to be posted for viewing by other users, a message to be sent to another user, a message board entry to be posted for viewing by other users, a document published and made available to other users, etc."
The document explains why it's a good idea: "Although advertising systems such as AdWords and AdSense have proven to be very effective tools for advertisers to reach a receptive audience, even automated systems that use sophisticated targeting techniques often can't match the effectiveness of manual targeting. However, manual targeting techniques don't scale well. Therefore, it would be useful to provide a scaleable advertising system that achieves at least some of the benefits of manual targeting. It would also be useful to provide a system of charges and/or rewards to encourage useful manual targeting of ads. Further, it would also be useful to track and use performance metrics of such ads if doing so would help an advertising system serve ads that are more useful. Finally, it would be useful to provide data structures and interfaces for enabling advertisers to participate in a system for manual insertion of ads into a document for distribution."
There's a discussion of this and two other related patents on Rough Type (via techmeme).
Earlier:
How to Advertise on Social Networking Sites
