One of my internships was preparing Bill Gate's demo machines for CES. I setup custom machine images and ran through scripts to make sure everything went off w/o a hitch (I was doing just the demos for Tablet PC, each org presumably had their own team preparing the demos!)

Not doing it live would've been an embarrassment. I don't think the thought ever crossed anyone's mind, of course we'd do it live. Sure the machines were super customized, bare bones Windows installs stripped back to the minimum amount of software needed for just one demo, but at the end of the day it sure as hell was real software running up there on stage.