I recently recorded a 16 minute screencast on how to securely publish Exchange 2003 OWA using ISA Server 2004. In order to make the screencast as easy as possible to record, I did simplify things by using only a single Exchange 2003 Server.
In my screencast, I opted to issue a web certificate to the Exchange Server and a separate web certificate for use on the ISA Server. In order to maintain an SSL connection from end to end, both the ISA Server and the Exchange Server needs to have a web certificate to secure the OWA connection. Instead of using two separate certificates, I could have used the same one for both. Using the same certificate for both does require a little bit more on the configuration side of things, but is possible.
Here is the link to the blog as well as the video.