I think the problem you're going to run into there is the passing of the map and everybody using the same software. I agree that GIMP seems to be a good default choice, but I use photoshop and can't say I'm really enthused about learning a new program, even one as supposedly similar to PS as GIMP is.
Very good point. It would be implausible to expect most people to learn a new software for the sole purpose of the game. If I do end up generating the interest, it will probably best to create a third position aside from the Historian and Overseer--a Cartographer. This makes more sense than the idea I had. While worlds are collective they are usually mapped by an individual.

To make this workable, I think the best route would be to use color-coded line drawings which can be edited using GIMP, MS Paint, PS, etc. and save them as .jpgs or something. This drawing should then be submitted to the Cartographer at the end of every round, or perhaps every turn, so that he or she can put it into a workable format. Great suggestion!

And it's too bad you don't have the time General, we would have loved to have you. However, if this thing takes as long to start up as I think it will, it will probably begin early next year. If you have time then, you are more than welcome to join in.