Last two months for some reason were pretty busy of calls for ISA Server issues where customers were running on a non supported scenario. Interesting enough, the articles for non supported configurations are out there since ISA Server 2004. Maybe it is time to refresh your favorites and to start add those articles to it. Here you will find supportability boundaries, limitations and unsupported scenarios:
Article: Troubleshooting Unsupported Configurations
Description: In the above article you will find nice explanations about some behaviors, including the reason why ISA Server does not support multiple default gateways.
Article: Best Practices for Performance in ISA Server 2006
Description: this article will explain the options to deploy ISA Server 2006 in a virtual environment.
Article: Configuring ISA Server 2004 on a Computer with a Single Network Adapter
Description: this article is also valid for ISA Server 2006 and it has the limitations and unsupported scenarios for ISA Server when running in a single NIC system.
Besides the official ISA Server TechNet Library articles, we (ISA Server Team members) are documenting in the ISA Team Blog behaviors that are expected. Here are the articles that were published so far:
Understanding By-Design Behavior of ISA Server 2006: Buffering and Streaming Web Publishing Rule Content
Understanding By-Design Behavior of ISA Server 2006: Using Kerberos Authentication for Web Proxy Requests on ISA Server 2006 with NLB
Files larger than 512MB are not served from cache after ISA Server firewall service is restarted
The tip for the IT professionals that are implementing ISA Server 2006 is to review those articles before start any deployment. I know how frustrate it is to build the whole infra-structure and when call to CSS to open a ticket get the bad news that the environment is not supported. Although this can be a frustrated experience, you should feel glad that this product has very known and public supportability boundaries. This helps you to understand what can and what cannot be done before start deploying your ISA Server.