The developer of Achieve Planner is Rodger Constandse and he is quite open to user input.
Your request is one he is working on right now. Achieve Planner will be syncing with Pocket Informant 2007, which was just released August 29, 2006. This should be complete in the December/January timeframe.
I have not used Pocket Informant but I know it supports hierarchical NAs and projects. So you could sync Achieve Planner with Pocket Informant 2007 and then see your hierarchical NAs and projects, as you created them in Achieve Planner, on your PDA using Pocket Informant 2007.
In the meantime, I sync Achieve Planner with Outlook. I then sync Outlook with my PDA. I do not see the hierarchical relations among NAs and projects in Outlook. But I do see my NAs and projects in their Contexts on my PDA. So I get by.
I do strongly recommend that if you are just starting out with GTD you keep it simple at first. Learning a new desktop application, a new PDA application, and GTD all at the same time would be too much for me, unless I had lots of leisure time. But if I had that much leisure time, I probably wouldn't be looking at GTD.
As I said, I started GTD with Excel because I knew Excel already. It takes some time to get comfortable with the GTD habit.
It should also be noted here that, from what I read on this forum, David Allen does not endorse creating hierarchical relations among NAs and Projects. I have the greatest regard for David Allen. So you should give serious consideration to his views. It has been my personal experience that using a hierarchical outliner like Achieve Planner has streamlined my GTD system. Just as David recommends using an outliner or mind-map to plan a project, I have found that it is useful to use an outliner or mind-map as my trusted system.
Good luck and let us know what you decide.