Can LIKE use indexes or not?

Reading the archives and the FAQ, it seems to be implied that LIKE can
use index (and ILIKE can't; so to do case-insensitive search you need to
create a functional index on LOWER(field) and say: LOWER(field) LIKE
'foo%').

However, EXPLAIN always says seq scan for the test data I'm using. I've
done 'set enable_seqscan to off' and it still says seq scan. I was
curious as to how the index will help this query:

db1=> set enable_seqscan to off;
SET
Time: 5.732 ms
db1=> explain select * from t where f like 'xx%';
                             QUERY PLAN
-------------------------------------------------------------------
  Seq Scan on t  (cost=100000000.00..100002698.90 rows=89 width=14)
    Filter: (f ~~ 'xx%'::text)
(2 rows)

db1=> explain select * from t where lower(f) like 'xx%';
                              QUERY PLAN
--------------------------------------------------------------------
  Seq Scan on t  (cost=100000000.00..100002893.68 rows=390 width=14)
    Filter: (lower(f) ~~ 'xx%'::text)
(2 rows)

The table is:

db1=> \d t
      Table "public.t"
  Column | Type | Modifiers
--------+------+-----------
  f      | text |
Indexes:
     "i1" unique, btree (lower(f))
     "i2" unique, btree (f)

It contains +- 250k rows of totally random 10-char-long strings
(containing upper- & lowercase letters and numbers). Here's how the LIKE
performs:

db1=> select * from t where f like 'xx%';
      f
------------
  xxEqfLZMkH
  xxBRRnLetJ
  ...
  xxFPYJEiYf
(98 rows)

Time: 452.613 ms

Would using an index potentially help the performance of this query, and
if yes, how do I force Postgres to use the index?

db1=> select * from t where lower(f) like 'mmm%';
      f
------------
  MmmyEVmfSY
  MMmzolhHtq
  ...
  mMMWEQzlKm
(16 rows)

Time: 634.470 ms

--
dave