Getting pk of the most recent row, in a group by

I've got a table of "coupons" which have an expiration date.  For each
type of coupon, I'd like to get the primary key of the coupon which will
expire first. 

# create table coupon
(       coupon_id serial primary key,       type varchar(255),       expires date
);
insert into coupon values(DEFAULT,'free','2007-01-01');
insert into coupon values(DEFAULT,'free','2007-01-01');
insert into coupon values(DEFAULT,'free','2007-06-01');
insert into coupon values(DEFAULT,'free','2007-06-01');
insert into coupon values(DEFAULT,'50%','2008-06-01');
insert into coupon values(DEFAULT,'50%','2008-06-02');
insert into coupon values(DEFAULT,'50%','2008-06-03');

The desired query would look like:

# select coupon_id,type,expires from coupon where type='free' order by
expires limit 1;coupon_id | type |  expires  
-----------+------+------------        1 | free | 2007-01-01


But be grouped by type:

# select type,min(expires),count(*) from coupon group by type;type |    min     | count
------+------------+-------free | 2007-01-01 |     4    ; pk=150%  | 2008-06-01 |     3    ; pk=5

In the second example, is it possible to get the primary key of the row
with the minimum expires time?

-- 
----
Visit http://www.obviously.com/