From Volker Will's blog:

The ISA Server Team will release an update to the ISA Server 2004 Firewall Client.  As part of the development process, they are running a short beta program to collect valuable feedback. 
Highlights:

• Support for the Microsoft Windows Vistaâ„¢ operating system (builds 5231 and later).
• Support for the the 64-bit versions of the Microsoft Windows operating systems.

If you are interested in joining this beta program, please follow these instructions:

1. Go to http://beta.Microsoft.com
2. Sign in using your Passport account details
3. On the home page, click the link below the welcome message ("If you were issued a Guest ID by Microsoft, you can sign in by clicking here")
4. Enter the Guest ID: ISAFWC_x64
5. Choose "Survey" on the left side of the page
6. Complete and submit the survey.