Apple has increased scrutiny on suspicious App Store reviews, and in some cases has already started removing them, according to a report from TechCrunch.
Short praise-filled five-star reviews are the norm in purchased reviews, which less than scrupulous developers purchase from less than scrupulous providers because highly rated apps with lots of reviews are more visible in the App Stores. Some services even go so far as to "downvote" similar apps in the same category to further game the system.
There's never any sort of "official" announcement on something like this, Apple never comes out and says it is laying the smack down on fake reviews, or any other sort of change, so there's no way to know if this is a new policy in place for everyone or a couple of isolated incidents.
TechCrunch did a bit of digging and discovered one case where twenty thousand reviews disappeared virtually overnight. That certainly seems like an outstanding case of purchased reviews, and given that the fake reviews are outstandingly fake, it's pretty obvious which developers are using services to goose the number of reviews. These fake reviews are usually not even an entire sentence (coherence optional), and if you look at other apps from the developer you'll probably see similar-looking reviews for dissimilar apps.