Contributed By: Cliff Hobbs [MVP ConfigMgr/ SMS]
As detailed in the "What is Decommissioned DDR Processing in SMS 2003 SP2?" article, Decommissioned Discovery Data Record (DDR) Processing is a new feature introduced in SMS 2003 SP2.

This feature is not enabled by default.  To enable it (you should only enable this on one Child Primary Site per hierarchy branch) simply set the value of the "DecommissionNotification" registry key at the following location to "1":

HKLM\Software\Microsoft\SMS\Components\SMS_Discovery_Data_Manager\DecommissionNotification