I use a palm m500 and sync regularly with my laptop (Sorry Merlin -- carrying around a stack of 3x5's and Moleskines seems complicated to me ;) .

My GTD system is pretty simple. I keep all my lists in plain text files. I use an app called MacNoteTaker which allows me to sync folders with text files on my Palm. The MacNoteTaker app on the Palm allows me to edit them.

I have two primary lists that I work from: Project List.txt and ToDo.txt.

Project List.txt is one big list of all the project I am working on, divided by context: @Home, @Computer, @At Work, @Errands. I started with more, but have learned that it's best to keep it simple.

ToDo.txt Is my daily ToDo list. This is the list I use most often. I reference at what I call "transitions": before I leave for work, before I head home, before I sit down at a computer etc. I also use it for capturing stuff that I want to sort through later. I have it divided into weekdays. I'll look at my calendar on Monday and put stuff onto the list. I updated it as needed throughout the day. Here's a simplified example

ToDo

TUESDAY
- Pick up kids at Y
- See what basement lights need to work
- Hang bikes in basement
- Fix bathroom sink
- Buy new eyebolt for heavybag

WEDNESDAY
- Teleconference w/JL at 11
- 2nd proof for DLP due

THURSDAY
- Lunch Dad meeting at Edison 7:30

SATURDAY
- Skating w/kids at 1

ERRANDS
- 6ft Ethernet cable
- Ceiling tiles for basement
- Cinnamon almond bread from Breadsmith

I have two sub-directories called Projects and Reference. Projecst are planning docs - a text file for each project. Reference is just that - text files of reference info.

An important lesson I've learned - be flexible and keep it simple. Don't be afraid to continually tweak your system. If you spend more time on the system than on getting things done, then something is wrong.