This weekend I discovered (quite by accident) a very simple way to enable shared sign-on across multiple Drupal sites running in a multisite configuration.

Suppose you have the following sites:

• phones.com
• htc.phones.com
• iphone.phones.com
• motorola.phones.com

The first requirement is that all the sites be running on the same domain. The examples above work — they're all subdomains of phones.com.

The second requirement is that all the sites share the same user tables.