Google announced "multi-domain support in Google Apps, that allows organizations with two or more domains on Apps to manage them from a single control panel. Users belonging to different domains within an organization keep their domain-specific email address but can see coworkers from other domains in the organization's global address book. It's also easy for users to share across domains in Google Docs, Sites and rest of Google Apps," explains Google.
This feature is available now to Premier and Education edition users at no additional charge. To learn more about multi-domain support on Google Apps, visit Help Center or join live webinar on July 13th at 9am PST / 12pm EST / 5pm GMT.