There is an installed base of Windows CE devices using SQL Mobile 2005 (which is what MS called it at the time) which is considered 3.0. The backend is SQL Server 2005, with the replication agent (sscerp30.dll ) installed on IIS.
Fast forward a couple of years, SQL Mobile is now called SQL Compact (thanks MS, so helpful) and the latest version of SQL Compact that will work with SQL Server 2005 is SQL Compact 3.1.
My question is, will the 3.1 clients be able to replicate to the server? It will take many days to get the resources setup to implement and (importantly) test so if it is known this will not work I can save a bunch of time.
To be clear, this is more about the IIS replication agent that handles SQLCE replication. I imaging SQL Server 2005 is blissfully unaware of the difference between 3.0 and 3.1?
The short answer is Yes.
The long answer is that supposedly 3.0, 3.1 and 3.5 can all replicate with SQL Server 2005/2008 (restrictions are based on publication compatibility level). Using Multiple Versions of SQL Server in a Replication Topology – Required Components and Compatibility Levels for Web Synchronization