We’ve already spoken about two methods of using databases in WP7 but here’s another one worth noting because it seems pretty powerful.
…there is a database available that WP7 Silverlight developers can use: it’s called Perst .NET which is developed by McObject and the source code is pure C# and freely downloadable. As distributed, it works fine with the full .NET Framework, with the .NET Compact Framework and with desktop Silverlight, and they have a demo app of a CRM that runs in the desktop browser in Silverlight (read about it here). Using Perst for .NET, developers using Silverlight can now include true database management system features in their Silverlight applications, including adding persistence to this data by storing it in a container file in isolated storage.
For the source code and more information on Perst .Net check out this thread.