Matt Neuburg, developer of the TidBITS News iPhone App, has put together a good description of how fast app switching works in iPhone OS 4, and explains why it might take some time for developers to add support for fast app switching to current applications.
“Basically I had to consider every state the app might be in at the moment the user comes along and suspends it. That turned out to be remarkably difficult – and the TidBITS News app is very simple and small! Imagine, then, how long it will probably take before your favorite third-party apps are updated.”