I see in some of the SL literature that SL4 (and possibly older versions) allow for file access on the local machine. Would it be possible, then, to install SQL Server Compact on the client machine, and have an SDF file that the Silverlight app could read?
Or I guess, rather, have the Silverlight app talk to a .Net DLL that's reading the SDF file, since Silverlight should be more on the "view" side of the framework.
Edit The reason for not going straight WPF or Windows client is that we have a use case where we want a web app that can go offline for a limited time. For example, traveling somewhere in rural Brazil that might not have internet connectivity (or bad connectivity.) In that case, we'd like them to take the same app and go OOB and run locally for a time, and then re-sync when they reconnect.