Drush failing

I just installed drush on Ubuntu 9.04. When I run the command: $ drush dl

I get this error:

Fatal error: Cannot clone object of class SimpleXMLElement due to 'zend.ze1_compatibility_mode' in /home/bigvortex/drupal/drush/commands/pm/pm.drush.inc on line 810
Drush command could not be completed.

This is what I have set in /etc/php5/apache2/php.ini

; Enable compatibility mode with Zend Engine 1 (PHP 4.x)
zend.ze1_compatibility_mode = Off
zend_debugger.allow_hosts=127.0.0.1
zend_debugger.expose_remotely=always

I have tried setting the mode to on but that doen't stop the error. I have searched but can't find anything on this error.