Sharing a VPN connection using Internet Sharing (via Airport) Internet Sharing from my MacBook Pro (10.6.6) to other WiFi devices works fine.

But the moment I turn on the VPN connection (PPTP) on the Mac (which I would also like to share), the other devices cannot access anything anymore (everything works fine on the Mac itself).

What do I do?
Linking to rodrigo.sharpcube.com is appalling. I lost hours of my life following it. It's a totally unnecessary wild goose chase into IPFW. Please avoid. The first answer from JRobert is accurate. Here is a more detailed step by step with images.