Few IT resources have greater visibility within an organization than email. It's a service that's critical to every user, and email outages cause the user base to panic. Migrating from one email system to another is always a difficult proposition, and the move from an in-house Microsoft Exchange mail system to a cloud-based system like Google Apps is even more challenging. Dealing with the inevitable postmigration problems can try men's souls.
The migration from Exchange to Google Apps is built on two tools: Google Apps Directory Sync and Google Apps Migration for Microsoft Exchange. The former is necessary to synchronize user accounts, groups, shared contacts, and user profiles in an Active Directory domain with Google Apps, and the latter does the grunt work of actually copying email, calendar, and contact data from Microsoft Exchange to Google's service.
The first order of business (once you've set up your business account with Google) is to begin constructing your Google Apps Directory Sync configuration. This requires deep knowledge of your Active Directory structure, as well as general knowledge of LDAP and its query structures.
Exchange to Google Apps: Google Apps Directory Sync