Here are just a few of the snags that I've run into while using this system. I put them here so that you can hopefully avoid them. If you run into any new ones, please email them to me so that I can keep track, and help others avoid them.
This error is associated with mis-matched versions off pppd. If you get it, try upgrading both ends of the connection to the latest version of pppd. I've found that pppd version 2.2 has this problem, so use version 2.3.7 or 2.3.8 instead.
This error is generated by route. I've seen it happen when the sleep time between ssh and ppd is not long enough. If you get this error, run ifconfig, and you may see that there is no pppX interface. This means that ssh was not done authenticating before pppd was launched, and therefore pppd did not make the connection. just increase the delay, and your problems will be solved.
I wonder however if there might be some pppd option that will fix this problem.
In the new 2.2 kernel, you must specifically enable IP forwarding in the kernel at boot up. This with the following command:
# echo 1 > /proc/sys/net/ipv4/ip_forward
Without this, the kernel will not forward any packets, and hence the server will not work, nor will any of the gatewaying clients.
It should go without saying, but be careful when you are routing real numbers that you don't route traffic destined for the VPN server's external address through the tunnel. It won't make it. (yes, this is from personal experience.)