Task Completion gives apps 10 minutes to finish whatever it is doing. It seems that certain apps are all but guaranteed to finish(Mail.app & Messages.app comes to mind). From my experience with my iPhone 3GS, it looks like if you get to the point where you are exceeding systems resources, the current app you are running will “crash”, but the apps in the background using the Task Completion api will keep chugging away for their given amount of time.