I've been having a lot of success with managing my action lists in an application called Projekt.

The pros:

Writes nice outlines, easy to read and use.
Exports as HTML or OPML, meaning you can work your lists on your desktop too, if you have an OPML editor (such as OmniOutliner).
You can manage your lists on the Memory Stick Duo (UIQ) or MMC (Series 80) and back them up for safekeeping easily (or use Bluetooth Send File function)
You may link items in outlines/projects to existing or new Tasks, Contacts, Appointments (sweet) giving nice context to contacts or tasks from your list.
Will prefix tasks created with context/project.

The cons:

No syncronization with desktop
Doesn't mark things as "complete" in Projekt if you finish it off in the Tasks application

I've been liking it so far. It means that my action lists aren't in iCal by default, but that seems to be okay so far, since iCal isn't granular enough for GTD in most circumstances anyway. You can create a top-level "task" on a per-project basis and check off the micro-tasks in your list, which means you can get a nice glance at the Tasks application to see what your overall projects are. Not too shabby. Could be better.

This is probably the best way to do GTD on Symbian devices, but there is no Series 60 port.

http://www.symbianone.com/content/view/2548/