Available here for download: http://www.microsoft.com/downloads/details.aspx?displaylang=en&FamilyID=e3303d34-af6c-4108-861b-dc05f9cf3e76
Keeping e-mail on the Exchange Server instead of allowing it to be scattered in Outlook Data Files (.PST files) helps reduce the risk of data loss, improve regulatory compliance, and increase productivity. With most email systems, the main barrier to implementing large mailboxes is the perceived cost and complexity of storage systems. Microsoft® Exchange Server 2010 is specifically designed to overcome these barriers without sacrificing performance or reliability.
This whitepaper talks about how Exchange Server 2010 is using storage more efficiently. It lays out the basics of disk performance, how IO makes a difference for Exchange Server and why larger disks are more efficient - resulting in larger mailbox capacities with Exchange 2010
An overview of resiliency with Database Availability Groups (DAG) explains how replication can be used to deliver robust storage systems at a lower cost.
Large Mailbox Performance – the User Experience
"While the management issues of larger mailboxes have been addressed by Exchange Server 2010, users still need to have acceptable performance. Larger mailboxes tend to contain more items. To ensure users had acceptable performance with Exchange Server 2007, it was recommended that folders contain no more than 20,000 items each. Thanks to database and indexing improvements, Exchange Server 2010 provides good user performance with folders containing up to 100,000 items."
"For users that want to reduce the number of active e-mail items in their primary mailbox, Exchange Server 2010 introduces integrated e-mail archiving and retention policies. This includes a new Personal Archive—a specialized mailbox associated with a user's primary mailbox. When using the archive, all e-mail data is still stored on the Exchange Server. The combination of the user's primary and archive mailboxes can be considered together as the user's large mailbox."