"fun with multipart primary keys" hobby kit

I tried searching the archives to find something like this. The search
function doesn't like me much, and believe me the feeling is mutual. So
I'm forced to pollute your inboxes with yet another "why the hell isn't
this thing using my index" email. I apologize in advance.

I have a many-to-many relation table with a multipart primary key:

siren=# \d listcontact
Table "public.listcontact"
Column | Type | Modifiers
----------------+---------+-----------
contactlistid | integer | not null
contactid | bigint | not null
Indexes: listcontact_pkey primary key btree (contactlistid, contactid)

(There were some FKs in there too, but I stripped off everything I could
during my investigation and they didn't survive.) I'm doing some
performance testing so I loaded it with a few elephant piles:

siren=# select count(*) from listcontact;
count
---------
1409196
(1 row)

And packed it down good:

siren=# vacuum full analyze;
VACUUM

I didn't get the performance I expected. I took one of our queries and
mutilated it and found some curious behavior on this table. I started
running queries on just this table and couldn't explain what I was
seeing. I tried this:

siren=# EXPLAIN ANALYZE SELECT * FROM ListContact WHERE contactListID=-1
AND contactID=91347;

QUERY
PLAN
---------------------------------------------------------------------------------------------------------
Seq Scan on listcontact (cost=0.00..29427.94 rows=1 width=12) (actual
time=893.15..5079.52 rows=1 loops=1)
Filter: ((contactlistid = -1) AND (contactid = 91347))
Total runtime: 5079.74 msec
(3 rows)

A seqscan... Fair enough, there's lots of memory on this box. I didn't
want to see a seqscan though, I wanted to see an index. So, I disabled
seqscan and tried it again:

QUERY
PLAN
------------------------------------------------------------------------------------------------------------------------------
Index Scan using listcontact_pkey on listcontact (cost=0.00..58522.64
rows=1 width=12) (actual time=402.73..9419.77 rows=1 loops=1)
Index Cond: (contactlistid = -1)
Filter: (contactid = 91347)
Total runtime: 9419.97 msec
(4 rows)

Am I reading this right? Is it only using half of the fully-qualified
pk index? How do I diagnose this? Has anyone seen this before?

postgresql 7.3.1
linux 2.6.0
quad xeon 450

chris