When I sat down to write this lesson, I was torn. Background tasks/multitasking is definitely the "must have" feature of iOS 4.0, but it's a challenge to demonstrate anything meaningful in the span of a dozen or two pages. What I hope we've achieved is a better understanding of how iOS multitasking works and how you might implement it in your own applications. Keep in mind that this is not a comprehensive guide to background processing—there are many more features available, and many ways that you can optimize your background-enabled apps to maximize iPhone battery life and speed.
As a next step, you should read the following sections in Apple's iPhone Application Programming Guide (available through the Xcode documentation): "Executing Code in the Background," "Preparing Your Application to Execute in the Background," and "Initiating Background Tasks."
As you review Apple's documentation, pay close attention to the tasks that your application should be completing as it enters the background. There are implications for games and graphic-intensive applications that are well beyond the scope of what we can discuss here. How well you adhere to these guidelines will determine whether Apple accepts your application or kicks it back to you for optimization.