The catch with iPhoneiGTD is that you need to be synching your iGTD data via .Mac. Like the impressive Webjimbo, which allows you to view, edit and create Yojimbo items over the web via a browser (with an iPhone-optimized option coming soon), iPhoneiGTD accesses your iGTD data by tapping into your .Mac account, though users are assured that neither your .Mac login nor personal info are recorded or stored.
As you can see, access is mostly limited to viewing your tasks via contexts and projects, though the plus button acts as a clever workaround for creating new tasks by allowing you to email yourself a task in the form of a subject + any notes you'd like to include. By setting up some kind of an automatic Apple Mail-to-iGTD trick like Brett Terpstra's Jott2iGTD, you could possibly turn this email function into a real automated solution if you leave Mail and iGTD running on a Mac somewhere. Otherwise, using iGTD's rich importing and keyboard shortcut abilities, you could easily process your tasks out of Mail manually once you're back on a Mac.
No matter how you slice it, iPhoneiGTD is a great solution both for GTD enthusiasts and those simply wishing the iPhone had some kind of a tasks app (assuming, again, that you have a .Mac account). Like iGTD, iPhoneiGTD is provided as donationware.