that's no longer the realm of VNC, that's something that would have to be supplied by the OS, not a third party (user space) app. Solaris does this, you can do it with X-Windows if you're cool. It's not something that VNC does. (to the best of my knowledge)
This is a feature that I'd like apple to offer, multiple simultaneous logins to a machine. The opportunity to zombie your login, login as someone else, do things, log out, log back in as you, and all of your apps are open and running just like you left them.
There are a lot of features that could easily be expected to be rolled into (or back into) Mac OS X. Remote windowing could come back. Remote login and simultaneous login would be nice. But what you're asking for is more of an OS feature than an application feature. It'd be some freaky security nightmare to have a third party app allow login as anyone after being launched as nobody special.