string_agg hanging?

Hoping someone will be kind enough to share how to write a query that
uses 9.0's string_agg with a subselect, something like...

select
m.id,m.subject,m.from_address,m.date_sent,m.size,string_agg((select
address as to_address from recipient r inner join message_recipient mr
on r.id=mr.recipient_id and mr.message_id=m.id and
mr.recipient_type='To'), ', ') from message m, recipient r group by
m.id, m.subject, m.from_address, m.date_sent, m.size limit 20;

Unforunately, that just seems to hang. So I'm using...

select
m.id,m.subject,m.from_address,m.date_sent,m.size,array_to_string(ARRAY(select
r.address from recipient r inner join message_recipient mr on
r.id=mr.recipient_id and message_id=m.id and mr.recipient_type='To'),
',') as to_addresses from message m, recipient r limit 20;

Which returns in under 4 seconds. Am I doing something wrong with the
string_agg query to make it hang?

Thanks in advance - I greatly appreciate any help you can offer.

jr