In iOS5 sdk, Compiler will take care of memory management (ARC). Suppose I had built my app using iOS 4.2 and had manually done all the releses. Now, If I am going to run that app on iOS 5, will compiler again try to insert its own memory management? In this case, won't it get crashed?
When you start new project using iOS 5.0 SDK, you now have an option to turn on/off ARC..If you are running an existing project which is designed using pre iOS 5.0 version, ARC automatically is disabled..See apple's documentation on transition to ARC