Index use in BETWEEN statement...

Hello pals, I have the following table in Postgresql 8.0.1

Mydb# \d geoip_block
Table "public.geoip_block"
Column | Type | Modifiers
-------------+--------+-----------
locid | bigint |
start_block | inet |
end_block | inet |

mydb# explain analyze select locid from geoip_block where
'216.230.158.50'::inet between start_block and end_block;
QUERY PLAN
----------------------------------------------------------------------------
-------------------------------------------
Seq Scan on geoip_block (cost=0.00..142772.86 rows=709688 width=8) (actual
time=14045.384..14706.927 rows=1 loops=1)
Filter: (('216.230.158.50'::inet >= start_block) AND
('216.230.158.50'::inet <= end_block))
Total runtime: 14707.038 ms

Ok, now I decided to create a index to "speed" a little the query

Mydb# create index idx_ipblocks on geoip_block(start_block, end_block);
CREATE INDEX

clickad=# explain analyze select locid from geoip_block where
'216.230.158.50'::inet between start_block and end_block;
QUERY PLAN
----------------------------------------------------------------------------
------------------------------------------
Seq Scan on geoip_block (cost=0.00..78033.96 rows=230141 width=8) (actual
time=12107.919..12610.199 rows=1 loops=1)
Filter: (('216.230.158.50'::inet >= start_block) AND
('216.230.158.50'::inet <= end_block))
Total runtime: 12610.329 ms
(3 rows)

I guess the planner is doing a sequential scan in the table, why not use the
compound index? Do you have any idea in how to speed up this query?

Thanks a lot!