You'll have to excuse me if I'm a little naive, I'm pretty new to ISA.
I'm involved in a project to set up two ISA Server 2006 servers in an array. We have two Internet gateways guarded by two front-end Cisco PIX firewalls. The idea is to keep the PIX firewalls as front-end firewalls and the ISA array will be the back-end firewall/proxy.
Is there anyway that the two Internet gateways could be utilised in this scenario for load-balancing/resilience?
Sounds like they have two separate Internet connections with two separate PIX firewalls. If that's indeed the case, first I would put both Internet connections on a single perimeter router, make the PIX's a failover pair, or active/active cluster if on PIX OS 7.0, configure whatever load balancing or specific routing is desired on the perimeter router, and install ISA as if it were a single Internet connection.
Then there are a number of ways, assuming the perimeter router is a Cisco, that you can use policy routing to direct the traffic to the desired Internet connection. You can use source or destination IP, IP protocol or TCP or UDP port, DiffServ which ISA can tag the packets with, amongst a number of other possibilities. I do something pretty similar to this at one site and it works great.