According to iPhone Hellas (link to English translation) more changes to the review system could be on the way in iPhone firmware 2.2. Screenshots indicate that when users decide to delete a program they will be prompted to leave a review.
Time will tell how this process will influence the final rating for apps. If only users that are unhappy or bored with an app's performance get the prompt, then developers could receive a disproportionate number of low marks. Perhaps users that leave an app on their phone for a specific length of time should also be prompted to review?
We'll let you know as more features of the 2.2 beta are leaked.