Seq scan with a like operator

Hi,
I don't understand why with a like operator, PostgreSQL 7.4.2 does not behave the same as with an equal operator :

Here's a table with about 200.000 tuples :
CREATE TABLE public.annu_pers2
(
nom varchar(50),
prenom varchar(50),
nom_int varchar(50),
....
) WITH OIDS;

CREATE INDEX personne_fk3
ON public.annu_pers2
USING btree
(nom_int);

======
Here are the two explain analyze requests :
select nom_int
from annu_pers2
where nom_int ='X'

Index Scan using personne_fk3 on annu_pers2 (cost=0.00..125.66 rows=31 width=11) (actual time=0.359..0.359 rows=0 loops=1)
Index Cond: ((nom_int)::text = 'X'::text)
Total runtime: 0.490 ms
-------
select nom_int
from annu_pers2
where nom_int like 'X'

Seq Scan on annu_pers2 (cost=0.00..5885.60 rows=31 width=11) (actual time=1213.594..1213.594 rows=0 loops=1)
Filter: ((nom_int)::text ~~ 'X'::text)
Total runtime: 1213.729 ms

Could someone give me a reason of the sequential scan on the request with like operator
(I know that without % a like operator is not usefull, but it was to restrain as much as possible probability of mistakes - the problem remains the same with or without %).

Thanks for your answer(s).

Luc