While I'm waiting for NirvanaHQ to get their beta to a relatively stable place, I have been exploring other GTDbaseed solutions. Since I've started down this track, things like being able to tag a task with multiple contexts and sync across multiple devices have become mandatory necessities for me.
With that in mind, I found todo.txt. This text based GTD app is brilliant. It uses Dropbox to sync a simple text file that you can edit directly using a simple syntax on a desktop/laptop computer. It is small, simple and fast. I seriously contemplated covering over to todo.tx. The only thing that held me back was the lack of a decent desktop based rapid entry facility.
I still however highly recommend you check it out if you are looking for a small simple GTD based system for Android.