Here is some information on what ISA does vs IAG that I thought would be useful considering security solution in the context of exposing internal web services, SharePoint and Exchange Server services to the Internet.
ISA Server 2006 Features General application access from Web-enabled clients when content-specific policy is not required. Protocol validation and filtering Pre-authentication OWA-specific content inspection Application and user-level policy SharePoint link translation Simple publishing wizards Intelligent Application Gateway 2007 Features (Now part of Microsoft Edge Server Security Solution and will be a part of Microsoft Forefront Unified Access Gateway )http://www.microsoft.com/forefront/prodinfo/roadmap/uag.mspx Highly customizable and differentiated application access based on user identity, content and file attributes, URL, and client security state. Comprehensive pre-authentication and single sign-on Application-specific data protection Block specific functions or areas within applications based on endpoint profile Endpoint security verification Client-side cache and session clean-up (Attachment Wiper) Multiple policy-based portal configurations with link translation Flexible and customizable portal experience with automated application launch Native SharePoint services support
ISA Server 2006 Features General application access from Web-enabled clients when content-specific policy is not required.
Intelligent Application Gateway 2007 Features (Now part of Microsoft Edge Server Security Solution and will be a part of Microsoft Forefront Unified Access Gateway )http://www.microsoft.com/forefront/prodinfo/roadmap/uag.mspx
Highly customizable and differentiated application access based on user identity, content and file attributes, URL, and client security state.
Forefront Edge Web - Secure Remote Access
Secure Remote Access ISA Server 2006 vs IAG 2007 - A Connected World through Software Architecture - Harry Pavithran - Site Home - MSDN Blogs