I have the web site rule set up but I am getting a certificate mismatch error. I have setup the rule using this walkthrough:  http://microsoftguru.com.au/2010/08/08/how-to-configure-reverse-proxy-using-forefront-tmg-2010-step-by-step/ (I'm aware of the typos)
As a test I have already established a working rule without using https and that seems to work fine. I am somewhat new to TMG and SSL/Certs so it could be that there is something wrong with my certs. I am using a server in my domain as a CA (Win2003) and I have not had issues with other internal sites.