Please remember that testing is a very important part of every site upgrade. It would not be difficult to write a script to copy code & db from live to stage for all sites, and then run drush -y pm-update on each site in turn; however, you should include a manual testing phase after the pm-update, and then push each tested site back to the live site as they are found to be working.
For the deploy step, you will want to copy the code from stage to live, and then run updatedb on the live site. The live site should be offline during this operation. See also drush_deploy for scripting stage-to-live deployment.