Postgresql, update if row with some unique value exists, else insert

This has been asked many times. A possible solution can be found here:
https://stackoverflow.com/a/6527838/552671

This solution requires both an UPDATE and INSERT.

UPDATE table SET field='C', field2='Z' WHERE id=3;
INSERT INTO table (id, field, field2)
       SELECT 3, 'C', 'Z'
       WHERE NOT EXISTS (SELECT 1 FROM table WHERE id=3);

With Postgres 9.1 it is possible to do it with one query:
https://stackoverflow.com/a/1109198/2873507