OK.  Let's say you only want to share users across a multisite installation.  In a plain Drupal installation, are these the only tables you need to share? (copied from the single signon module)

 *   'default'        => 'somesitename_',

* 'authmap' => 'shared_',
* 'profile_fields' => 'shared_',
* 'profile_values' => 'shared_',
* 'role' => 'shared_',
* 'sequences' => 'shared_',
* 'sessions' => 'shared_',
* 'users' => 'shared_',
* 'users_roles' => 'shared_',
* 'users_uid_seq' => 'shared_', // for pgsql
Anisa.

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