Thanx to Keith Alabaster I now have a functional tri-homed ISA server that is happy to allow routing between the two Internal network that the ISA Server is protecting. The only key element now is to allow only the DHCP broadcasts to pass between the two networks as this us currently not allowed.
Short and sweet, I need to be able provide computer in my LAN segment IP addresses from the DHCP server that is housed in the Server segment:
LAN Segment <-> ISA Server <-> Server Segment
For more information on the current configuration of the firewall rules: http://www.experts-exchange.com/Security/Firewalls/Q_21906673.html