How do I disable/enable a module on all sites within a multi-site Drupal installation using drush?
Drush has the ability to interact with each site within a multi-site installation via the @sites argument.  Anytime you are within the directory structure of your platform, the drush @sites command will target all sites rather than a single/no site.  In this way, any drush task can be accomplished across all sites.  

Some concrete examples:


Disable the dblog (core database logging) and devel module on each site 

drush @sites dis dblog devel -y

Enable the speedy module on each site

drush @sites en speedy -y

Change the admin theme for all sites

drush @sites vset --yes admin_theme "ember"

Check to see if any sites in the platform have the cdn module enabled

drush @sites pml --no-core --type=module --status="enabled" -y | grep cdn



These (and many more commands) work with any version of drush >= 4.x