The best you can do is a direct link to the review page in iTunes. So you ask them to rate the app and supply a button that takes them directly to the reviews page. See this previous StackOverflow answer.
Rate on delete used to be built into the operating system and forced upon everyone. It's now been removed. Many of us consider this to be a good thing, since the implicit question was "what do you think of this app, given that you don't want to keep it?".
EDIT: iOS 6 came out after this answer was first posted; Apple has added SKStoreProductViewController which allows you to show the iTunes page for any product directly inside your app. So you can remove the step that jumps out to an external display.