A new TechNet article giving an overview of Exchange 2007 namespace planning written by Patricia DiGiacomo Eddy from the Exchange UE team has been released.
"One of the most important decisions that you must make before you deploy Microsoft Exchange Server 2007 is how to arrange the namespace for your organization. A namespace is a logical structure that is usually represented by a domain name in DNS. When you define your namespace, you must consider the various locations of your clients and the servers that house their mailboxes. In addition to the physical locations of clients, you must evaluate how they connect to Exchange 2007. The answers to these questions will determine how many namespaces you must have. Your namespaces will typically align with your DNS configuration. We recommend that each Active Directory site in a region that has one or more Internet-facing Client Access servers have a unique namespace. This is usually represented in DNS by an A record such as mail.contoso.com or mail.europe.contoso.com."