Check out the thread I started here...
I'm currently finishing one software project and creating a system specifically for Allen's GTD will be my next one. It seems like there is a lot of stuff out there, but nothing I have found that truely maps to Allen's laid out procedures and is intuitive to use.
If another programmer writes it first, great. If it works great and my mind can flow with it, great, but until there is, I'll be building one of my own. I plan to release it to whoever would like to use it for a reasonable price, if not for free.
Tim, if you have any more ideas to add, mention them. I like the one about integration with GPS, detecting what store you're at and coming up with a screen of what you need to get there.