All environments are complex and differ in their requirements.
Generally, we recommend keeping OWA 2003 and 2007 environments separate during the migration by keeping the Exchange 2003 front-end/back-end servers intact. In this scenario, an Exchange 2007 CAS and Exchange 2007 Mailbox Servers support the 2007 users, in addition to the existing Exchange 2003 Front-end and Back-end Servers.
The redirection can be configured on the CAS where requests for /exchange are redirected to the Exchange 2003 front-end servers.
To further simplify the environment, we recommend putting an ISA server out front to act both as a firewall and a proxy server. Requests received by the ISA server are sent directly to the Exchange 2007 CAS or the Exchange 2003 Front-end Server depending on the where the user's mailbox resides.
Diagrammatically, here is what the migration model would look like, with ISA 2006:
Click image to enlarge
The ISA scenario is preferred because:
• It simplifies authentication and configuration
• It simplifies the management of the 2003 and 2007 environments