Aaaargh! No, it doesn't. It means NULL. Nothing else.
Well, x = UNKNOWN doesn't make any sense... the answer is UNKNOWN.x IS UNKNOWN does make sense, the answer is true or false. Replace UNKNOWN with NULL...
Actually it means what the DBA wants it to mean (which opens the door to many a misguided design...)
I hereby light a candle to the pgsql designers who didn't inflict 00-00-0000 00:00:00 upon us besides NULL dates.
