As always, when someone finds a great way to game the search engine, Google steps in to smack him or her down. This is no different than what is going on with paid reviews right now. Bloggers, in particularly, are being targeted as so are the three main third party networkers of reviews. Google wants bloggers to put the nofollow attribute on the paid review backlinks, so that the search engines ignore the reviews.
The advertiser, however, has paid to put the backlink on the review. So, instead of penalizing the advertiser, theyve decided to penalize the bloggers with paid reviews that did not use the nofollow attribute. Some blogs went down to a PR0 in a short amount of time. So, bloggers are now becoming quite wary of taking paid reviews from third party companies.
They are caught in the difficult dilemma of either taking a paid review and cheating the advertiser with a nofollow link or using dofollow and potentially blacklisting their own blog. This doesnt mean they cant do paid reviews, it means they have to be careful that a paid review alerts Google who penalizes them. This does not seem to be the case with other major search engines like Yahoo! and MSN. This is a recent development and one that is important to keep an eye out for as search engine algorithms change to adapt to changing markets.
If you are the advertiser and you buy a link that ends up being a nofollow, then you wont get backlink credit for it. In such a case, you may want to try to locate bloggers directly who will offer paid reviews with a little more discretion that can still give you credit and wont pull their own PageRank down. In the end, if either you or your sponsored backlink slides in PR, you both suffer.
Since some blogs were penalized down to 0, some advertisers started going back to pay these bloggers to actually remove their sponsored post, as odd as that may seem. Another way that might work for bloggers who are continuing to do reviews but dont want their PR devalued is to actively block certain pages on their blog from being crawled by Google while continuing to use the dofollow so other search engines give the advertiser backlink credit.