This proposal has now been accepted and is available in the incubator.

During the process of refining the proposal, Ralph Schindler suggested renameing the validator, and splitting it in two, to make it as verbose as possible (i.e. the name says exactly what it does). So, an abstract class has been added, and the DbMatch name dropped. The validators are  now called Zend_Validate_Db_RecordExists and Zend_Validate_Db_NoRecordExists. check the wiki page for more information.