Hello good people!

I don't know how to ask my question clearly so I'll just show you the money.

To start with, here's a sample table:

CREATE TABLE sandbox (
    id integer NOT NULL,
    callsign text NOT NULL,
    this text NOT NULL,
    that text NOT NULL,
    "timestamp" timestamp with time zone DEFAULT now() NOT NULL
);

CREATE SEQUENCE sandbox_id_seq
    START WITH 1
    INCREMENT BY 1
    NO MINVALUE
    NO MAXVALUE
    CACHE 1;

ALTER SEQUENCE sandbox_id_seq OWNED BY sandbox.id;

SELECT pg_catalog.setval('sandbox_id_seq', 14, true);

ALTER TABLE sandbox ALTER COLUMN id SET DEFAULT nextval('sandbox_id_seq'::regclass);

INSERT INTO sandbox VALUES (1, 'alpha', 'foo', 'qux', '2010-12-29 16:51:09.897579+00');
INSERT INTO sandbox VALUES (2, 'alpha', 'foo', 'qux', '2010-12-29 16:51:36.108867+00');
INSERT INTO sandbox VALUES (3, 'bravo', 'bar', 'quxx', '2010-12-29 16:52:36.370507+00');
INSERT INTO sandbox VALUES (4, 'bravo', 'foo', 'quxx', '2010-12-29 16:52:47.584663+00');
INSERT INTO sandbox VALUES (5, 'charlie', 'foo', 'corge', '2010-12-29 16:53:00.742356+00');
INSERT INTO sandbox VALUES (6, 'delta', 'foo', 'qux', '2010-12-29 16:53:10.884721+00');
INSERT INTO sandbox VALUES (7, 'alpha', 'foo', 'corge', '2010-12-29 16:53:21.242904+00');
INSERT INTO sandbox VALUES (8, 'alpha', 'bar', 'corge', '2010-12-29 16:54:33.318907+00');
INSERT INTO sandbox VALUES (9, 'alpha', 'baz', 'quxx', '2010-12-29 16:54:38.727095+00');
INSERT INTO sandbox VALUES (10, 'alpha', 'bar', 'qux', '2010-12-29 16:54:46.237294+00');
INSERT INTO sandbox VALUES (11, 'alpha', 'baz', 'qux', '2010-12-29 16:54:53.891606+00');
INSERT INTO sandbox VALUES (12, 'alpha', 'baz', 'corge', '2010-12-29 16:55:39.596076+00');
INSERT INTO sandbox VALUES (13, 'alpha', 'baz', 'corge', '2010-12-29 16:55:44.834019+00');
INSERT INTO sandbox VALUES (14, 'alpha', 'foo', 'qux', '2010-12-29 16:55:52.848792+00');

ALTER TABLE ONLY sandbox
    ADD CONSTRAINT sandbox_pkey PRIMARY KEY (id);

Here's the current SQL query I have:

SELECT
    *
FROM
(
    SELECT
        DISTINCT ON (this, that)

        id, this, that, timestamp
    FROM
        sandbox
    WHERE
        callsign = 'alpha'
            AND
        CAST(timestamp AS date) = '2010-12-29'
)
    playground
ORDER BY
    timestamp
DESC

This is the result it gives me:

id      this    that    timestamp
-----------------------------------------------------
14      foo     qux     2010-12-29 16:55:52.848792+00
13      baz     corge   2010-12-29 16:55:44.834019+00
11      baz     qux     2010-12-29 16:54:53.891606+00
10      bar     qux     2010-12-29 16:54:46.237294+00
9       baz     quxx    2010-12-29 16:54:38.727095+00
8       bar     corge   2010-12-29 16:54:33.318907+00
7       foo     corge   2010-12-29 16:53:21.242904+00

This is what I want to see:

id      this    that    timestamp                       count
-------------------------------------------------------------
14      foo     qux     2010-12-29 16:55:52.848792+00   3
13      baz     corge   2010-12-29 16:55:44.834019+00   2
11      baz     qux     2010-12-29 16:54:53.891606+00   1
10      bar     qux     2010-12-29 16:54:46.237294+00   1
9       baz     quxx    2010-12-29 16:54:38.727095+00   1
8       bar     corge   2010-12-29 16:54:33.318907+00   1
7       foo     corge   2010-12-29 16:53:21.242904+00   1

EDIT:

I'm using PostgreSQL 9.0.* (if that helps any).