It's an interesting one. We recently rebuilt our review system, because to be honest it was rubbish

We didn't allow negative feedback because when we did it was abused, and the positive was obvious self promo stuff.
So I spent a long time looking at other review sites, trying to find all the best ones, and went to speak to the guy that took on Trip Adviser and forced them to readdress their review system.
As a result, I think we made ours on My Local Services as fair as humanely possible, but the quality of review sites out there is amazing, and I say the majority we looked at were floored, with blatant unmoderated self promo, or legally worrying slander and abuse.
Paying someone to do fake reviews is no great surprise, but I suspect these will be pretty simple to spot, and no doubt a lot will be poor English
How you get a search engine to filter against bad reviews though is going to be a tough one, as Google is actively encouraging review based content as we move into a brave new social search world.