Automatic joins?

I tried posting this before, but I don't think it got posted. Sorry if
it did.

According to the Wikipedia article on PostgreSQL
(http://en.wikipedia.org/wiki/PostgreSQL), it has a feature for
automatic joins:

"SELECT u.* FROM user u, address a WHERE a.city='New York' and
a.user_name=u.user_name

In Postgres the relationship between users and addresses can be
explicity defined. Once defined the address becomes a property of the
user, so the search above can be simplified greatly to:

SELECT * FROM user WHERE address.city='New York'"

Is the a load of nonsense, or am I doing something wrong? I did define a
foreign key, but it doesn't work.