There is an available hotfix to be installed on ISA Server 2006 for publishing Exchange Server 2007 RTM servers.
ISA Server 2006 includes support for publishing the pre-release version of Exchange 2007. This software update adds support for publishing the original release version of Exchange 2007.
Get it from here: http://support.microsoft.com/kb/925403/en-us
Technorati : Exchange Server 2007 RTM, ISA 2006, hotfix