This is a hard question to research as there are "Linked Data Sources" in SharePoint Designer. NOT WHAT I'M TALKING ABOUT. :)
In Microsoft SQL Server you can create Linked Databases. I'd like to find a way to use a Linked Database as a Data Source in SharePoint Designer.
Why?
A common question in SharePoint Communities is how to go about adding an Oracle Database connection in SharePoint. I am attempting to discover a way of linking an Oracle DB to MSSQL and then connecting SharePoint Designer to MMSQL. I do this with the hopes I'd get more control over development in SharePoint Designer. Right now it is painful to use Oracle as a Data Source.
David Jenkins