Angelo Dinardi wrote a blog post about how his MailWrangler application has been rejected from the App Store for duplicating the functionality of a built-in iPhone application.
So in July I wrote a small iPhone app called MailWrangler. Basically this app enabled a user to add their GMail accounts (standard and Google Apps For Your Domain) which they could load and switch between them quickly. It embedded Webkit in to the app and logged you in to the accounts automagically. Normally to check multiple Gmail accounts in mobile Safari you would have to log in and out of all of the accounts, typing the username and password for each. Using just the Apple Mail application you arent able to see threaded views, your google contacts, archive (quickly), star, etc without going through the hassles that are present when using Gmails IMAP on the iPhone.
Apple rejected the application due to the duplication of functionality of the built-in Mail application "without providing sufficient differentiation or added functionality". Apple also pointed out some limitations in the function of the application.
Apple has generated significant criticism due to their rejection policies, though this rejection offers some more direction than previous rejection letters.