I am using the iPhone SDK 3.X to develop my application. But I want to get ready to 4.X. But there is some iPhone 4.X codes/API that can't not use in 3.X. So, how can I develop the same application for both 3.X and 4.X? Instead of making two copy of my application, can I have one source code to do support current 3.X and future 4.x? thank you.