Sub-optimal plan chosen

In the following query, We are seeing a sub-optimal plan being chosen. The
following results are after running the query several times (after each
change).

dev1=# select version();
version
-----------------------------------------------------------------------------------------------------------
PostgreSQL 8.2.13 on x86_64-redhat-linux-gnu, compiled by GCC gcc (GCC)
4.1.2 20070626 (Red Hat 4.1.2-14)

dev1=# EXPLAIN ANALYZE SELECT SUM (revenue) as revenue FROM statsdaily WHERE
ofid = 38 AND date >= '2009-09-01' AND date <= '2999-01-01';
QUERY
PLAN
---------------------------------------------------------------------------------------------------------------------------------------------------------
Aggregate (cost=11796.19..11796.20 rows=1 width=8) (actual
time=28.598..28.599 rows=1 loops=1)
-> Index Scan using statsdaily_unique_idx on statsdaily
(cost=0.00..11783.65 rows=5017 width=8) (actual time=0.043..25.374 rows=3125
loops=1)
Index Cond: ((date >= '2009-09-01'::date) AND (date <=
'2999-01-01'::date) AND (ofid = 38))
Total runtime: 28.650 ms

dev1=# set enable_indexscan to off;

dev1=# EXPLAIN ANALYZE SELECT SUM (revenue) as revenue FROM statsdaily WHERE
ofid = '38' AND date >= '2009-09-01' AND date <= '2999-01-01';

QUERY PLAN
----------------------------------------------------------------------------------------------------------------------------------------------------------
Aggregate (cost=13153.47..13153.48 rows=1 width=8) (actual
time=7.746..7.747 rows=1 loops=1)
-> Bitmap Heap Scan on statsdaily (cost=3622.22..13140.92 rows=5017
width=8) (actual time=0.941..4.865 rows=3125 loops=1)
Recheck Cond: ((ofid = 38) AND (date >= '2009-09-01'::date))
Filter: (date <= '2999-01-01'::date)
-> Bitmap Index Scan on statsdaily_ofid_sept2009_idx
(cost=0.00..3620.97 rows=5046 width=0) (actual time=0.551..0.551 rows=3125
loops=1)
Index Cond: (ofid = 38)
Total runtime: 7.775 ms

default_statistics_target = 100 (tried with 500, no change). Vacuum analyzed
before initial query, and after each change to default_statistics_target.

The same query, with a different "ofid", will occasionally get the more
optimal plan -- I assume that the distribution of data is the differentiator
there.

Is there any other data I can provide to shed some light on this?

Thanks!