Have you heard about SQL Server Everwhere and wonder what it is. Well Steve has put together a great Q&A about it
http://blogs.msdn.com/stevelasker/
In essence its SQL Mobile. It is hosted within your application and is essentially a data store. No support for Views, Sps, Triggers, SQL CLR etc.
So if you've always liked your database in files that are easily moved about and don't require a service i.e. like dbase dbf. Then this is for you. There is a real overlap with SQL Express on the desktop and comes down to what yuo want from your data store.