Stefaan Pouseele has done an exceptional job at solving the mystery of the Notify HTTP Users to use HTTPS instead checkbox. This checkbox worked the way you expected it to in ISA 2004, but no one was able to figure out why it would sometimes be available and sometime not available in the 2006 ISA Firewall. Well, you don't have to wonder about it anymore, as Stefaan has at least found a workaround. At this point, we have to consider this a bug in the user interface.
Read about Stefaan's solution at http://blogs.isaserver.org/pouseele/2007/03/18/require-128-bit-encryption-for-https-traffic-with-isa-server-2006/