This is a nice technical document that was released a couple of weeks ago: Exchange Server 2010 Design and Architecture at Microsoft, How Microsoft IT Deployed Exchange Server 2010.
With Exchange Server 2007, Microsoft IT streamlined the messaging environment through server clusters based on Windows Clustering, and a highly available, direct-attached storage technology helped ensure 99.99 percent availability. However, the costs and general limitations associated with the platforms and technologies used in the Exchange Server 2007 environment prevented Microsoft IT from efficiently meeting emerging messaging and business needs.
By replacing all servers running Exchange Server 2007 with servers running Exchange Server 2010, Microsoft IT created new opportunities to reduce costs and system complexities while increasing security and deploying new features not available in previous versions of Exchange Server.
- Increased reliability through new high-availability technologies, such as database availability groups.
- Increased mailbox sizes through Mailbox servers equipped with cost-efficient storage solutions.
- Reduced total cost of ownership (TCO) through cost-efficient storage solutions and elimination of backups.
- Increased compliance through transport rules, litigation hold, and Information Rights Management.
- Enhanced remote access and mobility options through Client Access servers.