For CribQ (housing maps from Craigslist) we prompt for ratings. It sucks to bug users but it also sucks to build a great product and not have people find it. To the people complaining about the experience, think of the experience of the developer to balance that feeling out.
We've also had issues with what seem to be targetted 1 star ratings with no reviews. Every new version that comes out we instantly get a bunch of these, and I have suspicions about who they come from. Almost all written reviews are 4 or 5 stars.
Have you looked into iOS 3 users? If I'm not mistaken, in iOS 3, deleting an app prompts the user to rate the app and if your update borks the app for them, they're probably going to delete and rate it one star.
We've also had issues with what seem to be targetted 1 star ratings with no reviews. Every new version that comes out we instantly get a bunch of these, and I have suspicions about who they come from. Almost all written reviews are 4 or 5 stars.