http://tombo.sourceforge.jp/En/
Tombo is a program for text notes that uses a tree structure similar to windows explorer. Another nice feature is that you can encrypt each of the notes (but only one-by-one, not the whole tree). Yet another nice feature is that you can run it on your desktop, too, and then sync it with the PDA.
Note: this is what is called an 'outliner' program, similar to treepad. Treepad is in fact a nicer program, and I am using it to write this site, but it is only for the desktop PCs.