LIKE on index not working

Hi all,

For some reason I just can't get this to use the index for the following
query. I'm using PostgreSQL 7.3.4.

Here's the details (let me know if you need anymore information to provide
any assistance):

Indexes: person_pkey primary key btree (personid),
ix_person_active btree (bactive),
ix_person_fullname btree (tsurname, tfirstname),
ix_person_member btree (bmember),
ix_person_supporter btree (bsupporter),
ix_person_surname btree (lower(tsurname))

smartteamscouts=# explain analyze select * from person where bmember = 1 AND
lower(tsurname) like lower('weaver');
QUERY PLAN
----------------------------------------------------------------------------
-------------------------------
Seq Scan on person (cost=0.00..12946.58 rows=310 width=416) (actual
time=873.94..1899.09 rows=6 loops=1)
Filter: ((bmember = 1) AND (lower((tsurname)::text) ~~ 'weaver'::text))
Total runtime: 1899.64 msec
(3 rows)

smartteamscouts=# explain analyze select * from person where bmember = 1 AND
lower(tsurname) = lower('weaver');
QUERY PLAN
----------------------------------------------------------------------------
---------------------------------------------------
Index Scan using ix_person_surname on person (cost=0.00..1265.78 rows=310
width=416) (actual time=0.91..2.03 rows=6 loops=1)
Index Cond: (lower((tsurname)::text) = 'weaver'::text)
Filter: (bmember = 1)
Total runtime: 2.36 msec
(4 rows)

As you can see, using the '=' operator it works just fine, but as soon as
the 'like' operator comes into it, no good.

Is this a bug in 7.3.4? Or is it something else I need to adjust?

Thanks for your help!

Chris