A quick note on this. I was troubleshooting a problem today of a newly setup Windows RRAS PPTP VPN server was not working. Or rather it was kind of working. You could connect and authenticate, but when it came time to passing traffic you could only ping the RRAS server itself. Which is a bit troublesome if you are wanting to access anything else on the network such as your file server, your domain controller, your Exchange server and so forth.
Capturing traffic via Wireshark did show that traffic from the VPN client would pass beyond the RRAS server and a reply would be sent. It just never makes it back to the client from the RRAS. Some quick queries to Google turned up little beyond more familiar problems of incorrectly configured multihomed RRAS servers. Which proved not to be the case here. It turned out that TCP offloading was rearing its ugly head again. After switching that off in the properties for the NIC in question traffic immediately started passing back and forth properly. This made for happy clients. So the moral of the story is probably that you should always suspect offloading no matter how fixed it is claimed to be. Or perhaps to use Intel NICs instead of Broadcom, but that remains as something that I will have to test out later if I get the opportunity.