Ryan McLeod, developer of the frustratingly delightful game Blackbox explains the careful dance of getting users to review your app:
Rather than a prompt, opt for an intriguing button, placed where it’ll catch the eye of someone in a mode of exploration—not the middle of editing or playing (Circa nails this). Remember, not everyone needs to be your advocate; fewer, but more exuberant reviews will quickly shadow many half-hearted ones. Fewer people will be nagged who wouldn’t leave reviews to begin with, and those who do will do so gladly.
Blackbox upends the usual iOS way of doing things in a number of ways, of which convincing users to rate the app is only just one. Ryan’s post is well worth a read, especially for developers trying to figure out how to get those reviews without raising users’ hackles.