You can combine setatakahashi's answer above with OS X's fast user switching, since you mentioned that you were prepared to first log into the machine you would like to control.
Log in to the machine you want to control. Fire up a VNC server like OSXVNC and set the port to 5902 or something of your choice that isn't the default 5900 used by the VNC server included in all Leopard Macs. (And which can be enabled by any admin user: Sharing pref pane=>Screen Sharing=> Allow VNC client to connect.) Set a password.
Enable fast user switching (Accounts pref pane=> Login options) and then, from the FUS icon in the menubar, choose "Login Window". The computer will then present Mac OS X's login window and allow others to log in and out normally without interrupting your session. (If the machine's preferences are set to display empty name and password fields, your active, hidden session will likely go unnoticed till someone logs in and clicks the FUS icon.) Your console session remains active, as does your VNC server. You can now control that session using a regular VNC client pointed at the right port on the remote machine. JollysFastVNC is a good, fast choice if your controller's also a Mac.
There are many security considerations, starting with a strong recommendation to use an SSH tunnel. Each account on the remote machine needs a password to stop users switching willy-nilly, users can kill your session by shutting down, sleeping, etc. Reliance on a wireless network might kill your connection, depending on how things are set up. The other stuff is probably outside the scope of this question. :)