I have been having a few problems with DomainKeys esp in Postfix but apparently its days are numbered and a successor is coming in the form of DomainKeys Identified Mail. I will probably move Postfix back to its previous configuration and have use of the DISCARD setting since DomainKeys will not gain popularity due to being replaced with something new.
Does anyone know how DKIM is coming along? It is the work of Cisco and Yahoo. 8)