Matt Neuburg over at TidBITS wrote a nice piece on the intricacies of Fast App Switching that does a wonderful job of explaining exactly what is expected by the user from iOS 4 on iPhone 3GS and iPhone 4 phones. I highly recommend anyone with an iPhone 3GS or new iPhone 4 to read this article as it will give you a much better understanding of how iOS 4 handles this.

An excerpt:

Those of you who have just used iTunes to install a shiny new iOS 4 on your iPhone or iPod touch, or who are about to obtain a shiny new iPhone 4, may find yourselves wondering about one of the touted features of this new system, fast app switching. What is fast app switching? What does it have to do with multitasking? Does it operate only through the new Home button double-press behavior? And why, for most apps, does nothing very remarkable or new seem to be happening?