In this article Damon Payne discusses the out of browser support in Silverlight 3, and also MEF and Isolated Storage.
Silverlight 3 ships with out of browser support which is a particularly useful feature. Combining OOB with the ability to detect network status can allow your users to consume your Web app without a connection. Combining both of these features with Isolated Storage and the right design approach can provide a seamless Occasionally Connected application experience using Silverlight 3.