After searching for a mindmapping software for Zaurus for a long long time, I finally came across vym and got the latest version (1.7.0) compiled. After testing and playing with it for a couple hours, I'd say it is an ideal all-purpose app for brainstorming, note-taking, outlining and practising GTD.
I took away the demo tree files and the manual from source to reduce package size. Still both are highly recommended: the demos give you a very good idea of different ways of using vym; and the manual is so well-written that you can master the app very quickly.
[Oh forgot to say it requires both zip and unzip, which can be found in the feeds, coz the .vym files are actually compressed archives of an xml with its image files.]