Rollback & Nextval fails - Mailing list pgsql-sql

I have been trying the following SQL code :

BEGIN;
INSERT INTO table VALUES ( NEXTVAL('serial'), 'Data' );
ROLLBACK;

And the insert function is rolled back but the serial sequence isn't. Hav I
misunderstood the functionality of rollback or is this a bug? Is there
someway to get the functionality that rollsback everything?

Cheers,

Fredrik