Concatenate aggregate?

I'd like to concatenate text from one field that has been returned by a
grouped query.

e.g.

create table test (id int, thetext text);
CREATE
playj=> insert into test values (1, 'Hello number 1');
INSERT 697997 1
playj=> insert into test values (2, 'Hello number 2');
INSERT 697998 1
playj=> insert into test values (1, 'Hello again number 1');
INSERT 697999 1
playj=> select * from test;
id |       thetext
----+---------------------- 1 | Hello number 1 2 | Hello number 2 1 | Hello again number 1
(3 rows)

I can do:

playj=> select id, max(thetext) from test group by id;
id |      max
----+---------------- 1 | Hello number 1 2 | Hello number 2
(2 rows)

But what I'd like to do is something like:

playj=> select id, concat(thetext, ' -- ') from test group by id;
id |      concat
----+---------------- 1 | Hello number 1 -- Hello again number 1 2 | Hello number 2
(2 rows)

[Ordering is unimportant here.]

Any pointers to useful starting points please?

Thanks

Julian Scarfe