All iOS VNC clients I have tried use regular VNC authentication. This is insecure because of the 8 character limit on the password, and so on. Some apps such as RealVNC's VNC Viewer, does have info panels that report whether the connection is encrypted, and it never says that it is, even when I turn both checkboxes in the System Preferences under Screen Sharing's
Computer Settings... off.
So, I am definitely taking the accepted answer to this question with a grain of salt.
It seems that the only way to go about this is to switch OS X's native Screen Sharing option off entirely, and then use some other method to host VNC securely. This is not a free option, though.