This simple hack for Joomla 1.5 is to change the following two files:

components\com_content\controller.php
components\com_content\models\article.php

For the first comment out some lines around 234:

// }

For the second file change a zero to one on line 336:

 

source: http://dzulkifli.com/index.php?option=com_content&view=article&id=75:how-to-allow-authors-to-auto-publish-in-joomla-15x&catid=37:joomla&Itemid=86