Troubleshooting "The command 'drush.php update' could not be executed."

I have several Drupal 5 Multisite installs. I was able to run "drush update -l www.example.com" on several of them successfully, but a few of the installs failed with "The command 'drush.php update' could not be executed."

Cryptic indeed.

There is a drush patch to provide more helpful error messages (http://drupal.org/node/449090).

but in my case it was just that the Drupal Core "Update Status" module wasn't enabled on the offending sites.

Many thanks to Owen Barton for mentioning this common error configuring Drupal 5 Multisite installs to use Drush in his patch writeup.