Wow - thanks everyone for such a welcome!
re the tutorial request. I did do a dog-rough tutorial for the folks on another forum, but searching this site I see you've already got a thread on the article that inspired me to try this mapping technique:
CG shaded mapping relief tutorial
Original Photoshop Roadmap tutorial
The bulk of the work was creating the greyscale DEM map from my existing (poor) maps - it took a lot of hours, and I couldn't have done it without a graphics tablet. After that, I just followed the Photoshop tutorial, adapting it to meet my needs.
re rivers: once you've got the greyscale DEM, drawing rivers becomes easy - just start a river on the side of a mountain and follow (on a new layer, obviously) the line of least resistance, moving most quickly from light grey to dark grey. With lots of starting points in the mountains, I found the rivers soon joined up into nice looking river systems.
I've yet to come to terms with bending text to a path - a quick tutorial on that would be very welcome!
One thing that disappoints me with this map is that when I tried exporting the greyscale DEM to Terragen (to see if I could get nice 3D images of valleys and stuff), the mountains came out like step pyramids - my DEM is too coarse to make realistic maps in that program, even when blurring the DEM in Gimp before exporting it. Something to do with 16bit vs 32bit greyscales - but I can't face the thought of having to do the entire thing again!
Here's a link to the complete map, for those who are interested.
Wow that is incredible. Man, I wish that I had a graphics tablet... Maybe for Christmas.
I covered bending text to a path using GIMP 2.4 in this post where I added labels to the terrain. That should be enough to get you going with the basic idea.