App Store reviews have been controversial from the beginning -- while they can be helpful for buyers, you often have no idea just who's leaving comments or what their real agenda is. Njection, the makers of Nmobile (which we played with a while ago) are having a huge problem with what they're calling "sockpuppet" reviews on the App Store.
Someone (they believe this person is in cahoots with their competitor) is posting bad reviews on their app and trying to trash them and their product elsewhere (including in a comment here on TUAW). And unfortunately, as they say, they don't really have much recourse against this behavior -- they've appealed to Apple, who've replied that they'll leave comments up, unless they're offensive or extremely false. Apple's own guidelines for reviewing apps asks that the reviewers deal with apps on their own merit rather than attacking competitors, but that seems to be more of a recommendation than a firm rule.
Njection says the comments have kept consumers from trying out their apps, though it seems difficult to actually track how many people haven't tried your app (and why). It'll be interesting to see if Apple makes other changes to the review system if this sort of thing rears its ugly head more often. At this point, it seems devs just have to deal with it by doing damage control when necessary and making their app good enough that "sockpuppeting" doesn't strongly affect public opinion.