While doing a clean install of Pfsense 2.1 this morning for a customer that will need VPN connectivity, I realized I have been using this OpenVPN tweak for a long time but have yet to share it. I originally found this tweak on the Pfsense forums here. Login to your Pfsense web interface and navigate to System > Advanced > System Tunables. Find net.inet.ip.fastforwarding and set the value to 1.
Note: this setting cannot be used with IPsec and does not generate ICMP redirects, the former is primarily why it is off by default.