I realize this probably isn't what the OP wants, but I use the outlining mode of Vim 7.0 on the N800 for any document more than a page long. You can see documentation at http://www.vim.org/htmldoc/fold.html.

By the way, for outlining, I use what is called "marker" folding in the documentation---one of five different folding methods, several of which are useful for outlining. You can open and close folds using screen taps on the N800 if you say something like

set foldcolumn=3

This will give you a graphical representation of the folds in the first three columns. If you also say

set mouse=a

you can then screen tap on the graphical representations for opening and closing folds. The marker method should work for writing in any language that allows comment characters, e.g., I organize my papers and programs as outlines, using LaTeX for papers and various languages for programs. Obviously Python programs already look like outlines so it's not needed for those. There are several Vim projects to make more complete outliners, but I have not kept up with any of them since the built-in thing does just barely enough for me.

As I said above, I realize the OP is probably looking for a program intended to be in the outliner genre, which Vim is not. I have purchased many, many, many of these outliner programs over the last twenty years and have archives of many, many, many inaccessible files in various formats, some of which are discussed at http://www.outliners.com/, a site that doesn't really go into the PalmOS products like Natara Bonsai, but does get to the general spirit of "information hiding" through outlining. Despite many great designs, I keep coming back to plain text as a long-term storage medium, hence Vim. I do have a couple of outliners on my Mac, OmniOutliner and Inspiration, and I keep expecting Chandler to appear with a cool outliner, but I doubt that I'll ever trust any of them like Vim with markers.