Hi, you're saying that you got all your other games to work. You don't then perhaps know how to make GameSpy Arcade work through ISA? This seems to be quite a mystery and I still haven't had any luck with it.
DUnno know if this would help at all but I did notice at home with my linksys gateway router that If my PC was connected to DiabloII in closed realm play then I could n't establish a PPTP VPN connection with my Laptop. That would imply either GRE47 is in conflict (unlikely), or that Blizzard is using som non-standard protocol to secure the realm connections that the linksys doesn't know how to route (more possible).
If the above is true then you may need to enable routing and and open those ports on the firewall rather than using a firewall client to essentially NAT you through. But then this is just theory based on 1 observation, best thing to do would be to sniff your machine when it tries to connect.