In earlier editions, Exchange server would direct Outlook to contact a Global Catalog server for user specific global catalog communication. In Exchange Server 2010 onwards, the Microsoft Exchange Address Book Service on the Client Access Server (CAS) hosts the NSPI endpoint. The Exchange Server 2010 CAS provides address book and related services to the Outlook client instead of referring Outlook to a global catalog server.
What does this mean? More Global Catalog communication from the Exchange server!
If you are still planning your Exchange 2003 to Exchange 2010 migration, make sure you factor this in especially if there are many users in remote sites. Users in remote sites will communicate with their local Global Catalog servers, when moving the users to Exchange 2010 these global catalog calls will no longer be distributed across the remote sites, all global catalog calls will hit servers in the same Active Directory site as the Exchange 2010 server which in large deployments can be a significant overhead especially when dealing with 10,000+ users.
As Exchange load balances its Global Catalog communication across all Global Catalog servers in the same site as the Exchange server as explained in my previous article "Find out which Global Catalog server Exchange is Using", the solution for this increase in Global Catalog communication is to simply deploy additional Global Catalog servers in the same Active Directory site as the Exchange server.