I give full credit for finally getting this to work to this Blog. In my test environment I have an Exchange 2003 server, an Exchange 2010 CAS/Hub Transport, and an Exchange 2010 DB server.
I have migrated some users to 2010, but most are still on 2003. I have made the CAS the entry point into the Exchange environment via mail.domain.com. I have kept the legacy OWA URL as webmail.domain.com.
To configure this run the following commands on the CAS (from the previously mentioned blog):
Set-OwaVirtualDirectory -Identity “prdcas01.domain.comOWA (Default Web Site)” -ExternalURL https://mail.company.com/owa -FormsAuthentication $True -BasicAuthentication $True
Set-ECPVirtualDirectory -Identity “prdcas01.domain.comECP (Default Web Site)” -ExternalURL https://mail.company.com/owa -FormsAuthentication $True -BasicAuthentication $True
Set-OwaVirtualDirectory “prdcas01.domain.comOWA (Default Web Site)” -Exchange2003URL https://webmail.domain.com/exchange
Next enables Forms based authentication on the Exchange 2003 server. To do that Open System Manager->Administrative Groups->Organization->Servers->Exchange Server->Protocols->HTTP and right-click the Exchange Virtual Server and go to properties. In the Settings tab check Enable Forms Based Authentication. If you have not enabled SSL, you will receive a warning.
Enable SSL in IIS on the Exchange front end.
Once you do this, users will be directed to the Exchange 2010 CAS when access webmail and will be presented with the Outlook Web App login screen. Exchange will then route the user based on their username to Outlook Web App or the OWA on the legacy server.