The reality is some developers pay for downloads and reviews to get higher rankings on the App Store. It's tough for Apple to do much about it since the sales look legitimate. It's easy to be frustrated by this sort of thing but there are two things you can do to beat it (ymmv, of course): 

1) Release a good app
2) Use Appirater

Appirater asks your users to review your app after some conditions are met. This seems simple but generally the folks who give feedback are the ones who are upset. When you ask repeat, often happy, users of your app to review it, you get some decent percentage of folks who are pleased to essentially /up vote/. #WINNER

We added Appirater to the latest version of BillMinder 3. Here is the current version's review graph: