Those dialog boxes are a double edged sword. If I really like the app then I will go and leave a positive rating in the App Store (Tiny Wings is one example). On the other hand, there was one app that explicitly requested that I rate it 5 stars (and there were a few other annoyances), so I gave it one star. Normally I'd just ignore or delete the app if it's bad, but if you pop up something and ask me to rate it, you better be prepared for the consequences.