jim. said:
I think the only way to do something like that in VNC would be to have a second X-session running in the background of your linux box. If you have X11 installed on your Mac, then you have other choices. One is to use ssh tunnelling with X11-forwarding, which would allow you to run individual apps from your linux box on your Mac. Another (not so secure way) is to use the remote X capabilities that are built into the X-server. To do this you would need to enable XDMCP on the linux box. Then you can point your X-server to the IP-address of the linux box and it will give you a full graphical login when it connects.
XDMCP is full of holes though, so you want to limit access using your /etc/hosts.deny file, and probably even use an ssh tunnel when you login. I would also not suggest doing this outside of a local network. If you want to know more about XDMCP, then a google search will give you loads of info.
Jim