In the past, I'd posted about SW's apparent review farms, where people left the same exact review on several books by the same author. I've found a new trick--apparently, Smashwords allows reviewers to leave multiple reviews, and then averages in all those star ratings.
I went looking for the highest-rated fantasy book. Currently it's
Blood Prize (Book #1 of the Running Veins Series) by Stella Purple, a vampire erotica book. (Um. Description is NSFW.) It has 69 five-star reviews, left by a total of 14 reviewers, all on the same day a bit more than 2 weeks ago--the same day it was published.
I don't imagine there's much SW can do about scams like "author asks facebook friends to give 5-star reviews," but they could limit the reviews to 1 per logged-in person.