What is need to upgrade an app from IOS3 to IOS4?

I have an app I have written for IOS3, but I want to polish it up for IOS4. What do I have to do differently (or in addition). I have noticed that every o开发者_如何学Gone in a great while, it will crash on the hardware at startup, but I haven't yet gotten a good debug session while it does it. I am hoping that the info received here will help me solve that problem also. (Note that I didn't have the crashes pre-IOS4.)

Thanks

Really, there isn't much that is required. If your build is clean of warnings and you have implemented applicationWillResignActive to save all your data you should be fine with iOS 4 and the multi-tasking features added. Supporting anything else isn't required to run on iOS 4...