This is correct. You just program for the target database, not the web service itself.
It is a data access layer. It is literally one line of code. If you want to swap between SQL Server and the ES (Enterprise Server) then you will just change the DataSource to use an EnterpriseServerDataSourceItem instead of a SqlDataSourceItem.
Program for your target database. It is literally a 5 minute implementation problem on the backend....that's the beauty!