From the iPhone Application Programming Guide:
Even if you develop your application using iPhone SDK 4 and later, you must still be prepared for your application to be terminated. If memory becomes constrained, the system might remove applications from memory in order to make more room. If your application is currently suspended, the system removes your application from memory without any notice. However, if your application is currently running in the background, the system does call the
applicationWillTerminate:method of the application delegate. Your application cannot request additional background execution time from this method.
applicationWillTerminate: will generally not be called very often in iOS 4. If you have to save data, you should do so in both