Google has published the description of a new feature to their "What's New" page for Google Apps admins. Your shared address book pulls together the complete list of accounts in your domain into an easily searchable list of contacts. The update isn't quite available for all the users in my domain yet, but I can enable contact sharing in my admin console already.
The Shared Address Book makes it even easier to get in touch with everyone in your organization without going to a separate list or company directory. When composing a new email, or sharing a document, you now have the option to search through all the users on your domain, even if they aren’t in your personal contact list. Need to send a note Paula S in Sales, but aren’t sure of her email address? As long as she has a Google Apps account on your organization's domain, all you have to do is type in the first few letters, "Pau … ", and your address book will help with the rest.
Unfortunately, it appears as though you cannot share email addresses outside your domain -- but most companies will agree that this feature is a great step forward.