Microsoft has confirmed the processing of AS 2005 cubes from SSIS 2005 invoked from SQL Server Agent defaults to serial/sequential processing, irrespective of whether the parallel processing option has been selected. This results in increased cube processing times (possibly up to 4 times longer) when compared with using XMLA scripted tasks from SQL Server Agent.
Microsoft has reproduced the problem and has escalated it to the SSIS Product Team.