Microsoft have released an update for ISA Server 2006 to support publishing Exchange Server 2007. You will also need to install this patch on any machines running the ISA Server 2006 management console:
This ISA Server 2006 update adds support for publishing Exchange Server 2007 with ISA Server 2006.
- The Attachment Blocking option is no longer available when publishing Outlook Web Access for Exchange 2007. If you are using Exchange Server 2007, we recommend you configure attachment blocking on the Exchange 2007 server. For more information about configuring attachment blocking on Exchange Server 2007, see Exchange Server 2007 product Help.
- This hot fix also updates the ISA Server Management MMC snap-in.
- Update for Publishing Microsoft® Exchange Server 2007 for Microsoft® Internet Security and Acceleration (ISA) Server 2006
UPDATE: The ISA Server product team blog has more detailed information on this update.