Limits in subqueries...

I have a (simplified) table looking like:

serial int4
custid int4
stamp  datetime

I want, for each customer, to get the serial of the the
highest timestamp for that customer, and for duplicate
timestamps, the highest serial. The way I thought how to
do that would be:

select custid, serial 
from test b 
where serial = 
   (select serial 
    from test a 
    where a.custid = b.custid 
    order by serial desc limit 1);

but thats not allowed. The best I could do is:

select custid, max(serial) 
from test 
where (custid,stamp) in 
   (select custid, max(stamp) 
    from test 
    group by custid) 
group by custid;

Which is ugly and only works in this case because
max does what I want. It has a horrible plan though:

Aggregate  (cost=1.36 rows=11 width=8)
  ->  Group  (cost=1.36 rows=11 width=8)
        ->  Sort  (cost=1.36 rows=11 width=8)
              ->  Seq Scan on test  (cost=1.36 rows=11 width=8)
                    SubPlan
                      ->  Aggregate  (cost=1.36 rows=11 width=12)
                            ->  Group  (cost=1.36 rows=11 width=12)
                                  ->  Sort  (cost=1.36 rows=11 width=12)
                                        ->  Seq Scan on test  (cost=1.36
rows=11 width=12)

In the more general case, this won't work.

Is there a better way of doing this? Does postgres 7.0 do
better in this case?
-- 
Martijn van Oosterhout <kleptog@cupid.suninternet.com>
http://cupid.suninternet.com/~kleptog/