BEFORE UPDATE trigger doesn't change column value

Hi,

Sorry for this newbie-question, I am trying for quite some time now to get
the following trigger-function to work properly:

CREATE OR REPLACE FUNCTION update_synced_column()
RETURNS trigger AS
$BODY$
BEGIN
IF NEW.synced IS NULL THEN
NEW.synced := false;
END IF;
RETURN NEW;
END;
$BODY$
LANGUAGE plpgsql VOLATILE
COST 100;

The idea is basically to always update the "synced" column to false, unless
a value has been provided manually in the UPDATE-clause.
Synced is defined as BOOLEAN DEFAULT FALSE;
I execute this function in a BEFORE UPDATE trigger FOR EACH ROW, however it
doesn't seem to have any effect.

Any ideas what could be wrong here?

Thanks, Clemens