I tried using Google Tasks for GTD, but found it didn't have enough features (no tagging, no way to delegate, no reminders, etc...). So I forged out on my own and created a task manager called GQueues (www.gqueues.com) that still integrates with your Google Account and Calendar (and mobile), but has the missing features necessary for GTD. Check out the video to see one way that it works: http://www.gqueues.com/GTD
Looks really great! - I just signed in and am going to check it out. Thanks!