This is a really well written article and does a great job explaining backgrounding on iOS.
Unfortunately, I've seen a couple of apps that have somehow been able to abuse the background APIs and have killed my battery. The only way to prevent this from happening is to force quit the app from the multitasking bar. The app I've noticed this with is GroupMe. If I don't kill it after closing it, my phone gets really hot and my battery dies. I have location turned off for GroupMe.
Unfortunately, I've seen a couple of apps that have somehow been able to abuse the background APIs and have killed my battery. The only way to prevent this from happening is to force quit the app from the multitasking bar. The app I've noticed this with is GroupMe. If I don't kill it after closing it, my phone gets really hot and my battery dies. I have location turned off for GroupMe.