In this article for the Visual Studio magazine Kathleen Dollard discusses how to store history information for the user on the local client PC in a sandboxed Silverlight 3 application.
Q: I'm writing a Silverlight 3 application to run in the sandbox. Can I store history information for the user on their local machine or do I need to pass this information back to the server for storage?
A: I'll show you how to store information on the user's local machine, but first let me warn you of the possible pitfall.