Ash was thinking about building something like this into Pligg as an option for Google Adsense. Here's a modified version of what the original mod says, only relating it to Pligg rather than Vbulletin:
This new feature will allow you to offer Google Adsense revenue sharing to your Pligg users. Revenue sharing is a great way to reward your members for their loyalty and adds a feeling of ownership to the site. Revenue is split between the Pligg administrator and story submitters. The percentage of probability is customizable by the administrator.
http://www.vbulletin.org/forum/showthread.php?t=80685