CiviMail cron job fails due to drupal bootstrap / session conflict

I recently upgraded CiviCRM from version 3.1.3 to version 3.3.1. I'm running Drupal 6.20.

I have always used curl to trigger the cron job for CiviMail, like this: curl -Ss 'http://<domain>/sites/all/modules/civicrm/bin/civimail.cronjob.php?name=<username>&pass=<password>&key=<key>

This had always worked before. After upgrading, civimail.cronjob.php fails with a fatal error. The error message is:

<b>Warning</b>: ini_set() [<a href='function.ini-set'>function.ini-set</a>]: A session is active. You cannot change the session module's ini settings at this time in <b>/home/outloudr/public_html/sites/default/settings.php</b> on line <b>134</b><br />
<br />
<b>Warning</b>: ini_set() [<a href='function.ini-set'>function.ini-set</a>]: A session is active. You cannot change the session module's ini settings at this time in <b>/home/outloudr/public_html/sites/default/settings.php</b> on line <b>136</b><br />
<br />
<b>Warning</b>: Cannot modify header information - headers already sent by (output started at /home/outloudr/public_html/sites/default/settings.php:134) in <b>/home/outloudr/public_html/includes/bootstrap.inc</b> on line <b>545</b><br />
<br />
<b>Warning</b>: Cannot modify header information - headers already sent by (output started at /home/outloudr/public_html/sites/default/settings.php:134) in <b>/home/outloudr/public_html/includes/bootstrap.inc</b> on line <b>546</b><br />
<br />
<b>Warning</b>: Cannot modify header information - headers already sent by (output started at /home/outloudr/public_html/sites/default/settings.php:134) in <b>/home/outloudr/public_html/includes/bootstrap.inc</b> on line <b>547</b><br />
<br />
<b>Warning</b>: Cannot modify header information - headers already sent by (output started at /home/outloudr/public_html/sites/default/settings.php:134) in <b>/home/outloudr/public_html/includes/bootstrap.inc</b> on line <b>548</b><br />
<br />Sorry, could not able to load drupal bootstrap.

(Actually, to see the details of the error I had to remove the "@" from the relevant line in CRM/Utils/System/Drupal.php).