Today I have finished development of 1-file version of "Getting tasks done!" service. It intended for iPhone & iPod Touch users. Application has "sync with server" feature. Note: if you want to use sync with server, you need to create account at GTD! and login.
If you want simple, reliable application for task management - here is it.
You may download it from Google Code:
http://code.google.com/p/offline-gtd/
Application is 1 HTML file with embedded css, images and javascript. No installation required.