I recently wrote that adopting PPP over SSL as common standard would be the best apporoach to standardasing SSL VPN. I must have been living on Mars: no other than Microsoft is already putting effort right into that!
They call it SSTP – the Secure Socket Tunneling Protocol. Microsoft RRAS team blogs quite a bit about it, and the beta is coming soon. The implementation is coming in Vista SP1 and Longhorn Server. I guess Microsoft will make it a standard in the Forefront Edge security products (next versions of ISA Server and the IAG) too. They should backport the protocol to Windows 2003 and XP. Microsoft won’t implement the protocol on Linux and MacOS, but that should be rather trivial; what can delay the implementation is closed beta.
I’ll look into testing SSTP ASAP.