This article will discuss how Outlook Web App will function in an Exchange 2003 or 2007 environment that has Exchange 2010 deployed. In order to introduce Exchange Server 2010 into your “Internet Facing AD Site” and support your Exchange 2003 mailboxes, you will move the primary OWA namespace that is associated with the Exchange 2003 Front-End servers and associate it with the Exchange 2010 CAS array. In addition, you will set the Exchange2003URL property on the CAS2010 OWA Virtual directory. Essentially, Exchange 2010 CAS does not support rendering mailbox data from legacy versions of Exchange. Exchange 2010 CAS does one of four scenarios depending on the target mailbox’s version and/or location.
Full Article: Exchange blog