For a simple free simulation such as Orbiter, there's no need for great acuracy. There's already a plugin for generating terrain from real or fractal functions. It's great because you can simulate a landing using realistic spacecraft parameters, velocities, mass, atmospheric pressure (if there's an atmosphere present), etc.
Here's more info and screenshots: http://www.orbiterwiki.org/wiki/Orulex
Please remenber that's it's a acurate simulation based on real world phisics and not an arcade starwars like game, so the visuals are not stunning. Nevertheless it runs very well on an average PC and in realtime.
At this time, only SRTM global Earth terrain , Clementine global Moon terrain and MOLA global Mars terrain are avaliable from servers. But it's possible to generate terrain from any grayscale image (less accurate but easier to do). That was what I was thinking about, a simple grayscale jpg with altimeter values on simple cylindrical projection would be very nice :-) ! Of course, there's already color mapped altimetry here:http://www.isprs.org/congresses/beijing200...s/4_pdf/172.pdf
but extracting them from the PDF would destroy any accuracy.