There is a neat article over at hothardware.com that goes into some of the changes to teh App Store, one of which just recently changed the Customer Review protion to only allow people who purchased the application to post a review.
Now it seems as though the App Store has changed the way applications under categories are listed. Before the change, developers would release an update to their application and it would then be shown first under a category. Some applications were getting updates pretty regularly, and then being bumped to the top of the category. And this was only because Apple was using “Last Updated” to rank the applications.
After the update Apple changed the App Store to place applications based on “Released Date”, instead of “Updated Date” so developers who continually update thier application are sorted correctly under categories.
You can read the full article at hothardware.com which also includes the author’s own graph of downloads when he updated his application.
As evidence of the phenomenon of peak sales for apps appearing on the first page because of updates, Vegesna provided the following chart for the sales of one of his apps (the peaks are from where the app appeared on the first page):
iPhone App Developers Gaming The System