Two years ago I put together revolutionary chart that detailed the flow of delegate messages when sending an app into background. This was even featured by Erica Sadun of TUAW because it for the first time visualized the whole process in a easy to understand way.
Based on this chart Carl Brown from PDAgent made an addendum to deal with the special case of background region updates and sent it to me. To my shame I have to admit that it was sitting at the bottom of my inbox for 4 months now and it is only now that in my effort to reach Inbox Zero I am dealing with this and putting it up here for your viewing pleasure.