Off-line scenarios with Silverlight is not something we currently support.
To do that, you would have to create your own layer that handles the caching in IsolatedStorage - that would be both tiles and layer metadata.
If you are offline, you won't even be able to start the silverlight application in a browser anyway. I guess that just leaves intermittent offline scenarios. If your application starts up online, you don't have to worry about caching the metadata information, but only the tiles. The nice thing about the tiles is that the browser already caches those for you.
Dynamic layers and featurelayers edits/query/update etc is a different matter though...