I am running a test upgrade, following the instructions on the wiki. This is on drupal 6.20 and PHP 5.2.10. When the upgrade script was called, it halted with the error below.
Fatal error: Call to undefined method stdClass::get() in /var/www/web/sites/all/modules/civicrm/CRM/Core/Smarty.php on line 120
I double checked the permissions on the civicrm directory in sites/all/modules. Then I decided to close the browser and reopen it, going back into the site through the drupal menus. That allowed me to access civicrm, and to run the upgrade script. It completed successfully and everything was back to normal.
Was this just a cache problem during the upgrade? It was worrisome but turned out ok in the end.