Why is it not using an index?

This must be really simple, but I just can't get it :-(
I have a table (a) with a single column (x):

Table "a"
Attribute | Type | Modifier
-----------+----------+----------
x | smallint |
Index: a_idx

Index "a_idx"
Attribute | Type
-----------+----------
x | smallint
btree

The table has 10000000 rows....

Now, how come, when I do:

explain select * from a where x=3;

it says:

Seq Scan on bset (cost=100000000.00..100175934.05 rows=303 width=2)

Why is it not using a_idx???

I even tried set enable_seqscan to off - makes no difference :-(

Any idea what is going on?

Thanks a lot!

Dima