I created a track using geomorph (natural heightmap), inkscape (track path + checkpoint coordinate finding) and gimp (merging of heightmap and track path for all 3 textures, color tweaks). It was quite fun and a smooth process, much more time was used on working on the level and testing it rather than fighting with the tools. :)
Here are some screenshots of the result: http://www.flickr.com/photos/qubodup/sets/72157628919692179/detail/
I don't think the level is very good to look at but it is fun and seems to differ from existing ones. Might feel more like a racing track rather than a rally one though. The color map feels pretty ugly to me. Should you want to include this level in the official release, it might be good to edit the colors.
I have seen .zip files in the svn and think that this method scares away from editing levels later (as it means a new revision of a fat binary data, rather than just one image or one txt file :) ).
I license the track under GPL of whatever version Trigger uses or if you want to use lgpl, cc-by3+ or cc-by-sa3+ instead, that would be fine as well. http://www.box.com/s/76oldk1yq1n54yxdf776
The 'source' (just the svg and png of the heightmap) is in public domain/cc0 http://www.box.com/s/vsdr9ki2uf30rj8vddgz