Select max(primary_key) taking a long time

Postgres 8.4.1
CentOS 5.4

I am trying to do
select max(primary_key) from some_table;

The explain looks like:
 explain select max(primary_key) from some_table;
                                                       QUERY PLAN
----------------------------------------------------------------------------
 Result  (cost=0.15..0.16 rows=1 width=0)
   InitPlan 1 (returns $0)
     ->  Limit  (cost=0.00..0.15 rows=1 width=8)
           ->  Index Scan Backward using some_table_pkey on some_table
(cost=0.00..161797059.16 rows=1086279613 width=8)
                 Filter: (trans_id IS NOT NULL)

I checked there wasn't a lock of any kind
select waiting from pg_stat_activity;
 waiting
---------
 f
 f
 f
 f
 f
 f
 f
(7 rows)

IOwait on the machine is around 20%
sar 10 5
Linux 2.6.18-128.el5 (trans05.afs)      05/21/2010
10:56:49 AM  CPU   %user   %nice %system %iowait  %steal  %idle
10:56:59 AM  all    5.90    0.00    2.04   20.67    0.00  71.39
10:57:09 AM  all    5.90    0.00    1.99   23.36    0.00  68.75
10:57:19 AM  all    5.87    0.00    2.10   22.56    0.00  69.47
10:57:29 AM  all    5.84    0.00    2.09   23.56    0.00  68.51
10:57:39 AM  all    6.30    0.00    2.23   21.53    0.00  69.94
Average:     all    5.96    0.00    2.09   22.34    0.00  69.61

Any ideas why the select would be taking long.. It has gone on for minutes
with no answer. I can just look at the value of the sequence for the primary
key, but I am curious why something that usually is sub-second is taking so
long..