This is an interesting problem. However self interest helps to reveal the problem. Google has successfully banned rings of cheap labour used to generate ad-sense income.
We had a conversation at work about how it would be much harder to stop people using distributed methods like this to attack people. By, for example, soaking up all your competitors' Ad Word budgets for the month with rogue clicks.
Yes, you can image ugly things with distributed methods like this. A problem in this case is that it is difficult to draw the line between what is still legal and what not. Paying people to add your web-page to delicious is probably not illegal. Although it kind of fakes interest for a web-site that may not exist, it is only a kind of indirect spamming with little effect on single users - probably even less harmful than many SEO techniques. As you said, interesting topic.
It probably breaches the terms of service which wouldn't make it illegal but it would be a civil offence. They could probably sue you for damages or loss of income (due to loss of legitimate traffic) or some such.
This is an example people gaming the system, and IMHO through legal channels. If search engines and social bookmarks integrate this content the burden is on them to filter it.
Please don't take my comments to imply that I condone spamming, but in this case I think a different resolution to the problem is in order.
We had a conversation at work about how it would be much harder to stop people using distributed methods like this to attack people. By, for example, soaking up all your competitors' Ad Word budgets for the month with rogue clicks.