Shouldn't be very difficult.  I have bought 2 flat screen TV's this winter and both have video inputs to use as a computer monitor.  Probably (you need someone with more experience than I have) you would want to put in a second video card and run a cable to the new screen.  Then to use it just switch inputs.  You should also be able to set it up as a second display so it would not necessarily be the same as the display on the desktop.

Maybe Scott at Farmworks coud give us a little advice.  I bet they do something like this on their show displays.