Constraint on row value based on values in other rows
As @NeilMcGuigan suggested, an Exclusion Constraint is the key here.  The part I was unfamiliar with was the ability to use the &lt;> operator.  As available post Postgres 9.0:

CREATE TABLE accrual (
    add_date date NOT NULL,
    user_id integer NOT NULL,
    rate integer NOT NULL,
    amount numeric(7,3),
    EXCLUDE USING gist
        (date WITH =,
        user_id WITH =,
        rate WITH &lt;&gt;)
);


Two helpful links are:


http://www.tutorialspoint.com/postgresql/postgresql_constraints.htm -- a short tutorial on constraints in Postgres, with an example for the Exclusion constraint that uses the &lt;> operator


and


http://www.slideshare.net/pgconf/not-just-unique-exclusion-constraints (specifically slide 23) -- a presentation on the need for and use of Exclusion constraints.  Slide 23 introduces the &lt;> operator and its usefulness.