I've signed up with an OpenVPN provider, and would like to share that connection across my home network (send all internet traffic out encrypted to a VPN server). I've tried this blog post and nothing in it got me to the point where the computer I was testing on could get to the Internet by using the OpenVPN connected computer as a gateway.
The idea is that I might connect to my ISP via ppoe on the Mac's Ethernet port and share it out (is this possible?) to my AirPort Extreme over wireless. (Maybe have the airport extend a wireless network created on the Mac?) Help me Obi Wans!
Update: I still think this is very valuable information to have, and I'm frustrated that it seems to be so hard to figure out, but after spending on the order of 10 hours on this and feeling like I was nowhere closer than I had been at the start, I went to flashrouters.com and bought a router already configured for my VPN provider. I'll probably use my airport express as a repeater. It was frustrating to pay for this when I know I should be able to get it figured out, but I anticipated spending another 10 hours on it at least, and in that amount of time, I can easily pay for the router doing other work. Please don't let this stop you from providing an answer, I just thought I'd leave a note about what I ended up doing in the end.