There are many things to keep updated on any server and Pantheon is no exception. Fortunately, we've provided some tools to help.
To update the ubuntu packages, run update_mercury.sh
To update the pantheon/mercury files, run update_mercury.sh (yes, it does both)
To update pressflow, run update_pressflow.sh
To update Drupal modules, use drush's update module option.
Hope this helps,
Greg