We've had a number of questions on problem with publishing Exchange 2007 through the 2006 ISA Firewall. I haven't had a chance to write about Exchange 2007 publishing through the ISA Firewall yet, as I don't have a processor on my workstation (Pentium D 820) that supports Intel's VT technology, which is required to run the 64-bit version of Exchange in a VMware virtual machine. I did try using the 32-bit version, but the SMTP "service" for Exchange 2007 said that there were insufficient resources and the "Hub" role wouldn't work correctly. I'm chalking this up to problems with the 32-bit demo version of Exchange 2007. But until I move to a machine with a VT enabled processor, I'm not going to be much help in your Exchange 2007 publishing scenarios.
That isn't to say there isn't help! Microsoft wrote a great guide on publishing Exchange 2007 and you can find it at:
I suggest that you read this guide very closely. There are a number of changes in Exchange 2007, especially on the authentication front, that you need to be aware of in order to get your Exchange Web Publishing Rules to work correctly.