Did you vacuum analyse your table after creating the index?
A solution for your problem would be to turn sequential scans ofd before querying the table.
This should work somehow like that:
myjava=# SET enable_seqscan TO 'off';
SET VARIABLE
myjava=# show enable_seqscan;
NOTICE: enable_seqscan is off
SHOW VARIABLE
check out the docs for further information.
Hans
