Exchange 2010 High Availability/Database Availability Groups. If you don't know Ross, he's a VERY senior Microsoft employee who wrote the Exchange Storage and Server Role Calculator. He knows Exchange, period. End of story. So, when he said that everyone should deploy Exchange 2010 in a multi-role configuration to improve performance and not break apart the roles, you need to take his recommendation seriously. This was the 1st time I had heard this. I had a long conversation with him about this in NYC, and he explained that for performance and the ability for leveraging failover capacity it is better to keep all the roles together. In theory, you could deploy less. Since if you were going to deploy 2 CAS and 2 Mailbox, you could in theory just deploy 3 consolidated roles. Well, Microsoft TechNet finally released some guidance on this. That only took 6 months. Don't forget to use a hardware/VM load balancer when deploy your multi-role Exchange Servers.
TechNet article title: Understanding Multiple Server Role Configurations in Capacity Planning
http://technet.microsoft.com/en-us/library/dd298121.aspx
-Ben