CREATE TABLE `test`
(
  `foo` varchar(32) NOT NULL,
  `datetime` datetime default NULL,
  PRIMARY KEY  (`foo`)
)
ENGINE=InnoDB DEFAULT CHARSET=latin1;

INSERT INTO test (foo) VALUES ('bar') ON DUPLICATE KEY UPDATE datetime = NOW(); # Works.
INSERT INTO test (foo) VALUES ('bar') ON DUPLICATE KEY UPDATE datetime = NOW(); # Works.
INSERT INTO test (foo) VALUES ('bar') ON DUPLICATE KEY UPDATE datetime = NOW(); # Does not work.

# The third time, it should update the value, but does not.