iNozbe provides full access to your tasks on your iPhone. A recent blog post by the developer details how the same interface can make a great dashboard widget. The only drawback I've found to the widget version is that the rare event that requires an alert dialog -- deleting tasks being the only one I've noticed -- doesn't work in Dashboard. But with iNozbe on my Dashboard and as GTD-to-go, it makes a great portable solution that can be accessed from anywhere. iNozbe makes use of a very complete API provided by Nozbe which can also be put to use in scripts and 3rd party apps for things like speedy task entry and getting your next actions in whatever form you want to process them as.
Given the lack of Mail and iCal integration a web-only task manager provides, it's not yet my ultimate solution; I'm still working on that. But, with an API which can update/export your projects and tasks, it can be worked into a system with applications like OmniFocus which provide AppleScript interfaces for task manipulation. Remember The Milk and others also provide such APIs, and the array of choices for both desktop and web tends to have me spending more time working on my "ultimate solution" for task and project management than actually getting anything done.
Nozbe has a free account with 5 projects available. Plans start at $2.45/month and stretch up to business accounts (starting at $24/month) which allow for group projects and task delegation. If you're in need of a new task management solution, have a look.