Rob Tiffany has a blog post here with links to the download and other required bits like SQL Server Compact 3.5 Service Pack 1 for Windows Mobile and Visual Studio 2008 SP1.
Synchronization Services for ADO.NET 1.0, which released with Visual Studio 2008, enabled synchronization between a server database and a SQL Compact 3.5 client database running on a desktop. The information at Rob Tiffany's blog provides details on how to obtain this functionality for device application development.
Download the documentation for Microsoft Synchronization Services for ADO.NET here. There is an article titled Introduction to Occasionally Connected Applications using Sync Services for ADO.NET that provides a nice overview of the Synchronization Services architecture as well as a Webcast titled "ADO Synchronization Services" available on MSDN.