Part 15 from Andrea Boschin's article series on Windows 8 Metro is already published at SilverlightShow.
From the author:
Until now in this series, I used a number of times the term "background tasks", but all that times I have only scratched the surface of this argument speaking about the results of their work instead of the way you can fully leverage them. It's time now to go deep on the topic, trying to fully understand the picture that is behind this matter. The background tasks strongly permeate a number of activities you can do, with the primary scope of making your application alive also if it has not been started by the user. But the real purpose of the architecture they are built upon, is to achieve the result of accomplish these tasks with the minimum consumption in terms of power supply, to prevent batteries from being drained rapidly. This is the main reason because, dealing with background tasks, poses a number of checkpoints that require a careful programming, to get them to work effectively and to avoid unwanted interruptions.
NOTE: The whole series will gradually be published into a multi-part ebook. Part 1, including the first 7 chapters from the series, is already available on SilverlightShow Ebook Shelf.