This is my first dips on the Drupal 8 database schema (from git version 10/19/2013). It took a new "standard install", and used mysqldump and MySQL Workbench to get the schema.
The relationships are not enforced in SQL and are my interpretation on how the tables relate to each other. Many times, the relationships are not exclusive - I connected entity_ids with node_ids - just one example of an entity that can get listed there.
The D7 variable system has been completely replaced by settings, the Configuration API, and the State/Key Value API (key_value table), but the variable table still gets created (but is empty in the standard install).
The table cache_config is the database representation of the Configuration PAI, and is in this aspect a special cache table, since it contains the content of the configuration yaml files in the files/config directory. This makes the cache_config table a special cache table, as its information is necessary to run the site even with regular caching disabled. You will find configuration settings for entity, field, views, system tables, etc.
I am sure the database tables will change significantly until D8 gets released. This one is a first snapshot. the MySQL Workbench file is attached, for anybody to play around.