Thanks for the replies, I've just downloaded and installed mu.wp so haven't looked at the code yet.
Just thought this would be something that would have been done, based on the popularity of wp.
Specifically what I've got in mind would be I would create specific locations where an advertisement would appear on various pages. The blogger would add their adsense/yahoo/? publisher id number in the blog control panel. Then when a page of theirs is viewed an advertisement would appear. I'm thinking of using a 90% ad share, meaning when the advertisement shows, 90% of the time it would have their publisher id in it and my publisher id the other 10% of the time.
By using their own publisher id, rather than channels or categories, if they do things they shouldn't do (clicking their own ads, etc) only their publisher account would be at risk and not mine. This would also give them the trust that I'm not 'fudging' the income as they would be able to log into adsense/yahoo and see earnings directly.
This idea is based on a modification available to vBulletin, where forum owners can share revenue with the users/posters of a forum. The concept has gotten a blessing from adsense, so I don't think their is anything stopping this from being done within the mu.wp community. But as I said, I just downloaded the code and don't know my way around it yet.
If anyone has any ideas about how this can be done or if it is being done, I'd appreciate any help with it.