I was just talking to a friend of mine, and apparently him and a group of about a half a dozen other webmasters have been running AdSense experiments recently. They were running AdWords campaigns on each others websites via AdSense, and then having other bystanders click their own ads to see where the chips fall.
The end results were that Google apparently bills advertisers for multiple clicks, but only pays out 1 click per user to the AdSense publisher [similar to Sedo's policy]. The other disturbing result, is that according to this batch of research, AdSense only pays out on average between 5 - 7% of what's billed to the AdWords advertiser, that's not including the substantial multiple click revenue they're pulling off the top as well.
Nice!