ISA 2006 Authentication Delegation

We have the following configuration

Windows 2008 R2 Server running Remote Desktop Host service

Windows 2008 R2 Server Running Remote Desktop Web Gateway

ISA 2006 Sp1

We are trying to configure SSO via user certificates on ISA .

So far we published the RDWEB server through ISA with user's certificate , but if we try to set the publishing using "kerberos authentication" in the rule it fails .

We have set the firewall server's computer account as trusted for delegation in AD .

Can anyone help with such configuration with an example ?

thanks