Thanks for the reply, gents.
Our network is pretty simple and flat. Yes, our real LAN is behind the ISA server. I'm using VMware vSphere to run both the ISA server and the pfSense box. The VMware hosts all have a direct connection to our public router, and the pfSense box has a dedicated public IP adddress for WAN – it doesn't go through the ISA server. I wanted to have our VPN users to be able to connect to the network, but I also wanted them to be subject to the rules of our MS ISA server (which is our current gateway) if they use the virtual machines on our network to go out. I can't have VPN users using our network to surf kiddie porn externally, for instance. When installing pfSense, I gave the LAN connection our ISA server as a gateway out of habit (I was and am still very new to pfSense) but it all seemed to work anyway, and like I said before, everything has been working great until I removed the LAN gateway. My firewall rules - OpenVPN tab has a list of rules that direct specific IP addresses (users) to specific virtual machines, and this has worked well to control access to servers on our network by the VPN users.