Entity Framework with File-Based Database
Mindscape LightSpeed has superb handling for file system databases (such as SQLite) and is free for small databases.
It's fairly straight forward to make the move as it has a LINQ provider, Visual Studio integrated designer (including supporting SQLite on the design surface) and there's significant help on getting up and running fast :-)
I believe NHibernate supports both SQL Server Compact Edition and SQLite. That is the route that I would recommend you take.
http://www.nhforge.org/doc/nh/en/index.html#configuration-optional-dialects