In this project Sergey Barskiy implements an Isolated Storage based database for Silverlight.
The usage of this software is very simple. You create a new database by Calling CreateDatabase. You can add tables to the database by calling db.CreateTable<T> where T is the type of entity to be stored in table rows. You can save database by calling Save, and open the database by calling OpenDatabase. This database supports version changes, so you can update your application and still be able to open the database. You can look for basic use of this database by examining test Silverlight application.