Seems Centos (thus RH) have changed the way things work slightly. Especially if you have multiple interfaces (NIC’s).
If your having problems with VPN not forwarding correctly, or your openVPN stopped working since updating from Centos 5, then this will most likely be what your looking for.
net.ipv4.conf.default.rp_filter = 1
net.ipv4.conf.default.rp_filter = 2
now reboot and enjoy.