Opting Out of Background Execution in iPhone OS 4.0

If you do not want your application to remain in the background when it is quit, you can explicitly opt out of the background execution model by adding the UIApplicationExitsOnSuspend key to your application’s Info.plist file and setting its value to YES. When an application opts out, it cycles between the not running, inactive, and active states and never enters the background or suspended states. When the user taps the Home button to quit the application, the applicationWillTerminate: method of the application delegate is called and the application has approximately five seconds to clean up and exit before it is terminated and moved back to the not running state.
Opting out of background execution may be preferable for certain types of applications. Specifically, if coding for the background may require adding significant complexity to your application, terminating the application may be a simpler solution. Also, if your application consumes a large amount of memory, the system might need to terminate your application quickly anyway to make room for other applications. Thus, opting to terminate, instead of switch to the background, might yield the same results and save you development time and effort.