The Google Strikes Back.
“Google wants to do a good job of detecting paid links. Paid links that affect search engines (whether paid text links or a paid review) can cause a site to lose trust in Google.”
So if you want to keep your prestige (and ranking / rating) with Google, you might want to re-evaluate the use of PayPerPost and ReviewMe type offers.
Some would argue about the issue of fairness, but in the free market economy of the WWW, Google can and should be able to set whatever policies it sees fit being in the dominant share of the market for user trustworthiness.
I suspect Yahoo and MSN do the same with regards to paid content such as PaidPerPost but have been waiting on word from Google to make their own declarations about these emerging platforms.