Informant for the iPhone has a task list that is switchable to the GTD style. i'm really loving it, but i'm also an avid Google calendar user. Google also has a task list in beta in their calendar. so far the tasks are not yet subscription based so i can't see my Google tasks on my iPhone *Informant* calendar, and i can't see my GTD tasks created in Informant on Google calendar. i'm sure its only a matter of time.
the great and powerful thing about Informant is that it gives you an interface almost identical with Google calendar, especially the month view. i love love love it.
i'm sure eventually they'll get the task's synced and then your Getting Things Done problem will be solved.
by the way, i also use Things on the iPhone--it is a spectacular task list app. it is my favorite, but does not yet integrate with Informant or Google calendar. someday soon i hope. i have contacted them.