Microsoft IT just released a new IT Showcase whitepaper that you might find very interesting. Here is what the problem they were trying to solve was, and the solution:
Microsoft IT ran Exchange Server 2003 mailbox servers in a clustered configuration to achieve high availability of 99.99 percent, but the Exchange databases remained a critical single point of failure, and the high costs associated with shared SAN storage hindered Microsoft IT from supporting employees with mailbox quotas of greater than 200 MB. The future Mailbox server design required a cost-efficient solution to support mailbox sizes between 500 MB and 2 GB. These increased mailbox sizes made it necessary to provide 10 times more data capacity on mailbox servers and implement redundancy at the storage level to eliminate the need for restores from backup as the primary recovery mechanism after a storage failure.
By implementing Exchange Server 2007 with a storage architecture based on CCR, Microsoft IT eliminated shared storage as a critical single point of failure by maintaining separate up-to-date copies of mailbox data at all times. This provided an opportunity to replace SAN technology with DAS equipment and support employee productivity with substantially larger mailboxes that accommodated higher redundancy levels.
This is a very interesting read, go here to get it:
A related TechNet Webcast: How Microsoft IT Implemented New Storage Designs for Exchange Server 2007 (Level 300) is also available. From that page, you can also download just the Webcast audio in WMA and MP3 formats.