/owa /exchange and other virtual directories in Exchange 2007: The key difference between CAS and FE is that CAS contains the business logic and renders the OWA UI. This presented us with a challenge: how do we allow Exchange 2007 CAS servers to coexist with Exchange 2007 Mailbox and Exchange 2003/2000 Mailbox servers? The solution depends on how the server is configured.

Continued: http://msexchangeteam.com/archive/2007/02/07/434523.aspx