Official Google Enterprise Blog: Introducing multi-domain support in Google Apps

Multi-domain support is a new admin control that allows organizations with two or more domains on Google 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 the rest of Google Apps.