This post of Jeremy Likness is intended to introduce you to Sterling (an object-oriented database that Jeremy created) and also to provide guidance for using it on the Windows Phone 7.
Source: C#er : IMage
Sterling is an object-oriented database that I created to facilitate ease of serialization of data in both Silverlight 4 (soon 5) and Windows Phone 7 projects. Sterling was designed with a few goals in mind. It is not a true relational database and not intended to replace one. I'm confident certain offerings such as SQLite or even a trimmed down version of SQL Server will make their way to the phone eventually, backed by experienced teams and highly optimized code.