Updating a date field

I have a date field called "updated" with a default of now.
When I INSERT data it the field displays today's date.
My problem is when I UPDATE the row it doesn't change unless
I update the field manually like this:
UPDATE person SET email='doe@foo.com',updated='now()' WHERE
person_id=614486;
Is there an automatic way?
I tried doing it with a rule, which returned
ERROR:  query rewritten 10 times, may contain cycles
I tried a CONSTRAINT and TRIGGER without any luck either.
Can anyone help?