aggregate that returns array

Hi,

Has anyone considered creating an aggregate function that returns an 
array of all matching rows?  I am not sure if this makes much sense from 
a speed point of view or is possible, but it would help denormalizing 
tables when necessary.  For example, consider a table that looks as follows:

SELECT * FROM t;id | value
----+------- 1 |   1.5 1 |   2.5 1 |   3.5 2 |   4.5 2 |   5.5
(5 rows)

It would be nice to be able to do a query as follows:

SELECT id, agg_array(value) FROM t GROUP BY id;id | agg_array
----+----- 1 | {1.5,2.5,3.5} 2 | {4.5,5.5}
(2 rows)

Thanks,
David Kaplan