Fortunately, Apple has decided to change the review process. Starting today (well, I noticed it for the first time today, and so did Matt Gemmell), you have to actually own the application in order to review it. Go ahead and try to enter a review for an app you haven't downloaded or purchased, you'll get the dialog box at the top of this post.
This is a GREAT step to making the App Store more equitable for both users and developers. Certainly, it doesn't make up for all of the other valid complaints about the NDA and Apple's non-transparent app review process, but it does address the issue that arguably can have the most impact on application sales.
Although free apps will surely still be littered with reviews that solely exist to complain or try to promote another product or continue Internet wars like we're all 12 years old, I don't expect BS reviews to continue to appear on paid apps. After all, is someone really going to pay to trash a competitor's app? This means that the reviews, both postive and negative, can become more reliable, and that would-be customers have a better chance of actually parsing their opinions before making a purchase.