Check out the new OpenEdge Principles design documents that have just been posted that cover the Data Access Layer:
The Data Access Object - http://www.psdn.com/library/entry.jspa?externalID=1571&categoryID=293
The Data Source Object - http://www.psdn.com/library/entry.jspa?externalID=1575&categoryID=293
The documents also come with related source code samples.
Happy Reading!