There are many rules to develop apps for the iPhone. If you don't obey the rules, your app will be rejected. Sometimes Apple reject apps because they don't work. Apps that run as background applications are not allowed. Safari plugins, apps that contains pornography, apps that copy the iPhone/iPod touch functions, apps that execute code from a server for example(not sure about this one) aren't allowed too. You can find the contract with the rules on the iPhone Dev Program Portal. Hope it helps. 😉