Tiger Fast User Switching
June 15, 2005
“In Tiger, Mac OS X 10.4, fast user switching gets a related feature. When a user session is switched off-screen, if a screen watching program such as OSXvnc-server is running, the off-screen session will get a virtual framebuffer so that it can be remote-operated while another user session or a login window is on the hardware console.”
So now you can remotely control a Mac while someone else is logged in as another user. [via]
Update: More information available at macosx.com