Just created a quick ERD for CiviCase, and shared it on this page http://wiki.civicrm.org/confluence/display/CRMDOC42/CiviCRM+ERD+3.3.
It is version 3.3, so not the latest and greatest. But I am sure I will have to check the same ERD for version 4 at a near point in the future and update the ERD too. And I do not think there are major differences in the data model......