Hi,
Even though I'm not a GTD user, I consider myself a bit of a phone nut so I'll try to throw my best advice out there.
Of the three major platforms, each has it's own benefits:
iPhone - Has the most options and tools available to you, but you may be overwhelmed with the options before you find the one you like. Reading over impartial blogs reviewing these apps may help.
Android - Phones running android (like the
Droid or
Nexus One) will integrate with gmail, google docs and google tasks better than any other phone will. These phones are designed to work tightly with all of google's tools, so if you find that your GTD system works inside Google's system to begin with, this may be a good indicator that an Android phone would be best.
Palm's WebOS - Phones like the
Pre and Pixi that use the fairly new WebOS have less apps to choose from (a few more than on android, actually, but still not an overwhelming number) which may mean that your favorite option isn't available just yet, but it's under heavy development. New apps are flowing into the less crowded app store, so new options are always coming out, and in 6 months or so, the operating system should be on par with iPhone in terms of quality and 'refinedness'. WebOS also puts apps in "cards" letting you see all of your apps running at the same time, move them around and focus on each as necessary. It sounds like nothing important, but it has a very zen feel to it that makes crunching work on your phone very relaxed, almost at ease.
I just did a quick search in Palm's app store. No apps were tagged with GTD, but my brother and I just released a new note taking / reminder app called
Jotter that's gotten several 5-star reviews from users who use it to Collect new items into their GTD system. It works by recording short notes you jot (type) into it, and sending them to your email address, where you can sort them into the system you already have in place. It's available on Android and WebOS, but not iPhone (yet).
I can't make a perfect recommendation, since it's hard to say exactly what you're looking for, but I hope that this post helps in some way, as you try to make your decision.