The answer for the curious, is no.
Other than using an external digitizer to send video in the Firewire or usb port, there is no way to play live video or a monitor from another source on the iMac screen. You could show another computer screen attached via a network using VNC or Timbuktu software
And to anticipate the next question, neither the digitizing solution or the software solution would be anywhere near fast enough for gaming or for most graphical user interface programs.