ISA 2006 without service pack has problem reading SAN DNS names, and this is the reason you need to have External web farm FQDN as subject name of the certificate which is configured on the default web site of the web component server.
This issue has been addressed in ISA 2006 service pack 1. ISA 2006 sp1 is scheduled to be release soon.
Reverse Proxy as an OCS Edge Role (Troubleshooting Reverse Proxy isues)
Configuring reverse Proxy through ISA 2006:
New Features in ISA 2006 sp1
The ISA 2006 sp1 is released and available fro download at
The ISA 2006 sp1 is released.. follow the following link to download it..