Having used both Palms and PPC's over many years - I started with one of the first "Pilots" a 500, I think, I've found that the new version of ListPro from Ilium Software works extremely well with GTD since it's both powerful AND flexible. Generally I found with most Palm and PPC apps such as DBK, Agenda Fusion, LifeBalance, Bonsai, Shadow, etc. was that they did not handle complex multi step projects very well or come with a Desktop program. ListPro has enough built in flexibility that you can design your own system. It's not perfect by any means since it doesn't really integrate with any calendar but it does support Start, End and Due dates that you can filter on. The power lies in it's hierarchal structure and ability to create unlimited custom categories, flag and check off items and maintain multiple separate project "lists". I'm using ver 4 Pro which comes with a companion PC desktop app that syncs with the PPC or Palm, which is great since it makes setting up lists and entering a lot of stuff much faster. In a Listpro list, each item can have many categories making it easy to assign items, etc. then filter the list to see next actions, etc. The categories show up as columns associated with each item and the columns can be sorted, filtered and hidden - very flexible. You can design your own list templates and re use them and Ilium has a number of pre built lists on their site. Anyway, I've set up my version as follows:
Each "project" has it's own list that is easily created from a custom template. The columns on each list are a Done checkbox, NAS (next action step - this is a "flag" column, Project Name (category column), Due date, Priority, Assigned to, In Progress, etc. I also have a Misc TODO list that I use for non project related tasks.
I have about 10 project lists going on at any one time. Each day I review each list. This doesn't take long since ListPro supports a hierarchal outline structure. I "flag" the next action step(s) for each project. Then I select the flagged items (Listpro has an automtic filter for this) and use the "copy to list" function to copy them all at once to my Daily TODO list. Now I can see all of my Next Actions in one place - the Daily TODO list which has the same columns as the project lists so I still see all the information associated with each item. Now I can evaluate all of the next actions in the context of "Today". I can prioritize or simply delete these off the Daily TODO list if they aren't going to get done today, my original project list remains the same. As I go through my Daily TODO's I simply check them off, mark them in progress, etc.. At the end of the day, I go back to my project lists and check them off on each project list (they're easy to find since they are flagged) and unflag them - (Yes, It would be nice if these lists could all be linked but I like the idea that I can delete items from my Daily TODO list without affecting anything else). For me the key is at the end of the day, my projects are all updated and I delete everything from my daily TODO list whether it got done or not - remember I still have those Next Actions flagged in esch project list - so it's incredibly simple and quick to start over the next day.
Using this system, each project gets tracked and I only focus on the Daily list which only contains Next Actions. I re evaluate my Next Action Steps for each project on a daily or weekly basis. If a new task comes up and it is obviously a Next Action, I just flag it as I enter it.
I know this sounds complicated but it's actually very simple and I've tried pretty much everything else out there. The flexibity to allow you to adapt it to your needs is a huge plus.
As a bonus, you can also get pre built lists with all kinds of information, use it to easily build a shopping list application, etc.
Last Comment: Download the manual - it's excellent and you need it to understand all the nuances of the program which despite it's simplicity is very powerful. You could easily add a column to the project lists to track associated files whether they're paper or links to files on your PC or URL's
Try the trial at http://www.iliumsoft.com/site/lp/listpro.htm