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.

  1. Protocol validation and filtering
  2. Pre-authentication
  3. OWA-specific content inspection
  4. Application and user-level policy
  5. SharePoint link translation
  6. 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 )

Highly customizable and differentiated application access based on user identity, content and file attributes, URL, and client security state.

  1. Comprehensive pre-authentication and single sign-on
  2. Application-specific data protection
  3. Block specific functions or areas within applications based on endpoint profile
  4. Endpoint security verification
  5. Client-side cache and session clean-up (Attachment Wiper)
  6. Multiple policy-based portal configurations with link translation
  7. Flexible and customizable portal experience with automated application launch
  8. Native SharePoint services support

Forefront Edge Web - Secure Remote Access