Is Apple's Screen Sharing tool secure?

Question

I've been using Apple's screen sharing tool to connect from a client to an Xserve for a while and it's just occurred to me that doing so might be quite a bad idea!

I know VNC is inherently insecure and that it is possible to tunnel it over SSH, but does Apple's Screen Sharing tool use any secure transport protocol? If not then surely I've been typing in server passwords on my client and they have been transported over the network in plain text.

Anyway, I now use the Server Management tools and SSH when possible, but I'm sure there will be times when I still need to use VNC.