A couple of blog entries back (http://civicrm.org/blogs/mbriney/contacts-erd-v31 to be precise) Matt Briney was kind enough to share the contacts ERD with us! I promised to put some more on. The entire CiviCRM ERD is too big and complex to be useful in this format, but I have just created one for the location point of view.
http://www.ee-atwork.nl/files/images/civicrm_location_erd.png
The ERD was created with MySQL Workbench (http://dev.mysql.com/downloads/workbench/)