We have recently blogged about our plans for a new feature of GAL Segmentation with Microsoft Exchange Server 2010.
Since the early days of Exchange Server our customers have used various methods to provide different views of the Global Address List (GAL) to subsets of users within the same Exchange Organization. This segmentation allowed administrators to optimize address lists in large companies, creating smaller 'virtual' organizations, with users only able to see the users they need to see. This segmentation still allowed users to share common features in Exchange such as Public Folders, Transport Rules and High Availability constructs.
Read more at http://msexchangeteam.com/archive/2011/01/27/457820.aspx