One of the latest developments in the SQLCE space is the beta for ADO.NET Sync Services. Sync Services ships as a standalone download, runnable on Visual Studio 2005 and is also built-in to Visual Studio Orcas. Both versions require a new version of SQLCE, version 3.5. This runs side-by-side with SQLCE 3.1, although both version's DLLs have the same names they are installed in different directories. The Orcas B1 version has a newer version of the DLLs and install GUI support.
The Orcas GUI support consists of a new component "Local Database Cache" that can be added a various types of C# and VB.NET projects. This bring up a Configure Data Synchronization dialog. In this dialog, you can configure a Server connection and a Client (SQLCE) connection, select "Cached data objects" (tables to be replicated) and replication specifics. You can also configure the feature of Sync Services that allows you to make synchronization service-based, that is, use a middle-tier WCF, Web Service, or other middle-tier component for connect to the database and perform the synchronization.
Besides service-based synchronization, Sync Services allows you to sync to data sources other than SQL Server. You can sync to most any database that supports ADO.NET, for example, Oracle. It works by instanciating a DataSet (although I'd though it could/might use EDM in future, see next post) and performing synchronization through the DataSet. Sync's DbServerSyncProvider uses a SyncAdapter, making it even more ADO.NET-like. Although DbServerSyncProvider can use any compliant ADO.NET provider, SqlCeClientSyncProvider is SQLCE specific.
For some excellent examples of Sync Services in action, check out The Synchronizer's (aka Rafik Robeal) blog. I think Rafik is up to six samples now, one of which works for an Oracle backend database.
For some excellent videos showing Sync Services with Visual Studio Orcas, check out the links on Steve Lasker's blog. Although I haven't tried it out yet, I think his WCF service-based example is running from the Orcas Beta1 version of the designer.
Next: SQLCE and Entities