We're running into a lot of questions on exactly the CAS redirects traffic. Here's what I know: It becomes a proxy for all 2003 Mailboxes during coexistence with Exchange 2010/2003. External traffic is seamlessly pushed to the 2003 front-end and mailboxes FROM CAS.
How does it route internal traffic?
Also, is it possible to install CAS without it touching the 2003 environment and simply doing a DNS change to switch it over to our production environment?