For information here is one of these dialogs: http://arashpayan.com/blog/2009/09/07/presenting-appirater/

This is quite configurable and you can decide to show the dialog only after a given amount of time and a number of uses. This makes sure that the user has used the app quite a bit.

Interestingly, users reacts quite well to this. If it annoys them they can discard this dialog and they would waste less time than by putting a 1-star review as a "punition".

This is especially helpful because updates reset the number of reviews displayed on the App Store. So you can drop for hundreds of 4-5* reviews to a few reviews just because you fixed a bug in an update.