We've been getting a lot of questions regarding mixing versions of SQL Server and SQL Server Compact for Merge Replication.  Laxmi wrote a very extensive post explaining all the details here.  As noted, this is for Merge, not Sync Services, but has lots of useful details.

http://blogs.msdn.com/sqlservercompact/archive/2007/12/19/connectivity-cross-version-compatibility-sql-server-compact-3-5.aspx

 Steve