If you need a relational database or if your application currently uses SQLite and you want to port it to Windows Phone 7, this post of Rudi Gobler might be very helpful.
Source: Rudi Grobler in the Cloud
C# Sqlite Port for Windows phone 7 and possibly Silverlight 3, 4. The core engine was slightly modified to be used with IsolatedStorage and SqliteClient were ported by using missing codes from Mono project in order to maximize usability and portability from desktop