Thomas Shinder wrote a two part article series about how to redirect OWA users to the correct directories and protocols. That article series was originally written with ISA Server 2004 in mind and can be found at:
With ISA Server 2006 the solution to redirect the HTTP requests to SSL (HTTPS) requests is very easily implemented by properly configure the Connections properties of the Web Listener as explained in Tom's blog Redirecting HTTP Requests to SSL Requests using the 2006 ISA Firewall. One of the solutions to redirect the users to the /Exchange folder was to include a special path mapping translating the root path "/" to the special Exchange path "/Exchange\". However with ISA Server 2006 this is no longer possible because it generate the Event ID 21177 as mentioned in my previous blog ISA Server 2006 and KB917025 and the following ISA Alert:
One possible solution is to include the root path "/" in the OWA web publishing rule and place the following 'default.htm' file in the root directory of the OWA site:
Note: you can download the above OWA_default.htm sample page at http://users.skynet.be/spouseele/OWA_Default_htm.txt. Of course don't forget to change the URL to what is appropriate in your specific environment.