Does iPhone iOS multitasking drain my battery life?
The official line from Apple is that iOS 4 can be trusted to properly manage background functions and minimize or eliminate battery drain. Most apps on the app switching dock are not using system resources even though they have been recently used. Some are in a suspended state and can be quickly restarted from the app switching dock.
Users have reported a different story, depending on which version of iOS they are running on what Apple device. Recent reports have included accusations that Apple doesn't police developers when it comes to their apps properly turning off background functions during sleep. The contention is that apps could increase battery drain even when the iPhone is sleeping due to programming or bugs.
There's only one way to be sure what's going on with your iPhone battery. Test the device without apps running in the background by removing apps from the multitasking dock on your iPhone. You'll get a better idea if your apps are causing a reduction in battery life.