I am referring to the popup window that asks the user to submit a review/rating.
I know it can be done since the Aardark app does it...it asks several times in fact!
I would check out the Appirater project that Arash Payan has put together.
Pretty much handles the checking and displaying of the "rate my app" prompt, and brings the user right into the review portion of your app on the App Store.
It's a very clean, and elegant way to provide that minimum barrier so that your users are more likely to submit reviews of your app.
Hope this helps...