There's a very simple tutorial for building basic apps using a background agent to update the pinned tile and pop up toast. The tutorial, by Dina Berry, allows the application will talk to the agent through an isolated storage file that is protected by a named mutex.
Need more info, here's a look at the architecture:
The app and agent don’t actually talk to a web service as this is a sample but in a real world solution they both would. The app would do the heavy lifting. The agent would have a quick, light conversation with just enough information to update the tile and pop up toast with meaningful information.
In this sample, you can configure the agent to run repetitively every minute, second, etc. This is usually done for debug/testing. In the usual release situation, the agent runs on the 30 minute cycle.
The .XAP binary and code solution for the tutorial are both available for download.
Make sure you read both parts of the tutorial in these two separate posts: