There was a bug in Joomla! version 1.5.16 that required some users to manually edit the configuration.php file to change the Session Handler parameter from "None" to "Database".
After you upgrade to version 1.5.17, if you like you can again use the "None" option for this parameter. Just go to Global Configuration→System and change the Session Handler parameter back to "None". Note that you will be logged out of the back end when you save this. However, you will be able to log back in as normal with this setting.