Which database engines will allow me to GRANT/REVOKE on a specific column?
SQL Server 2000, 2005, 2008 has this capability

GRANT { ALL [ PRIVILEGES ] }
       | permission [ ( column [ ,...n ] ) ] [ ,...n ]
       [ ON [ class :: ] securable ] TO principal [ ,...n ] 
       [ WITH GRANT OPTION ] [ AS principal ]


ADDED

In SQL Server 2005, 2008 it is possible to encrypt a column of data by using symmetric encryption, see B.Symmetric encryption that includes an authenticator. This variant returns a NULL value, just as if the wrong key was used.