Read the Cable Guy article about the upcoming support in Windows Server 2008 for SSTP (Secure Socket Tunneling Protocol) VPN connections e.g. VPN using SSL.
Unfortunately, I haven't been able to dig up any information about client OS support. Maybe Vista SP1 has it?
Update: Thanks to Steffen for explaining that the abbreviation is SSTP not STTP :) Likewise, he told that Vista SP1 will support SSTP.