Using advice from Drush: Update core from stable to dev I ran
drush up drupal-7.x in order to switch from 7.7 to 7.x-dev.
However I'm now in the position whereby I want to move the project back to 7.8, however I don't seem to be able to figure out how to do this. I've tried:
drush up drupal-7.8
Name Installed version Proposed version Status Drupal core 7.x-dev 7.x-dev Specified version available
So it appears to understand I'm asking for a new version, but following this through just updates the site to the latest 7.x-dev.