I upgraded a client system from 4.4.12 to 4.5.8 at the weekend and now message templates are not appearing when a user creates an email (regular or bulk). You can pick a template but nothing appears. In Administer/CiviMail/Message Templates all the templates are there, and can be edited & saved. My other upgrades to 4.5.8 have not shown this problem so I am puzzled - I've cleared the Drupal caches and CiviCRM caches.
I found an answer for my issue: the Pogstone Module (drupal) was in direct conflict. Removing that module has fixed the issue.