enable dd() for drush - temporary://debug_drupal.txt call fails
As of a recent change to Drush (March 30, 2013), Drush is aware the  environment variables TMP and TEMP.  So the solution (on Ubuntu) is:

export TMP="/tmp/"


and 

sudo chmod g+w /tmp/drupal_debug.txt
sudo usermod -a -G www-data DrushUser