I have written a GTD application for S60 phones. It's called MobileGTD and can be found on
http://code.google.com/p/mobilegtd/

It's very GTD centric and does not have a lot of extra functionality, but it does its job very well. Here is the description from the web page:

MobileGTD turns your Series 60 phone into a GTD notebook on steroids Features

• Based on a simple file and directory structure (easy synchronization with PC)
• Use your favourite file explorer to browse your actions
• Usable as a simple inbox, allowing you to create projects on the fly
• Searchable project list view
• Project editing functionality (add/remove actions/infos)
• Processing functionality (automatically find done actions)
• Review assistance (find projects with no active next action and projects that have not changed for a certain time)