Bumped into this by accident when looking up information of Form authentication on Exchange 2003. It looks like a nice program and rather cross platform. It also look like there is a front-end component to really secure VNC and/or RDP.
I know there are a lot of commercial apps out there to do this already, and to a certain extent, Squid, but this one is nice and free, for now.
Orenosp is an HTTP/HTTPS secure reverse proxy, load-balancer and secure port forwarder that runs on Windows platforms (NT, 2000, XP and 2003), Linux x86, and Mac OS X.