Update: See Read More on Dvorak for iPhone for more!
Unfortunately, the iPhone Dvorak keyboard hack isn't as good as it could be. There are two problems with it. First, the key clue is Cyrillic, not English. By "key clue," I mean the key that the keyboard displays above your thumb. When you press a key, the key is displayed above your thumb, and the keyboard doesn't type the key until you raise your thumb. This improves your typing accuracy because your thumbs obscure the keys, and you can move your thumb around the keyboard until you find the key you want, then release. While I can touch type Dvorak, touch typing doesn't translate directly to thumb typing. I need the key clues. The key clues, though, are Cyrillic, so they are useless for English.
The other problem is the lack of autocompletion. When I type English words on the QWERTY keyboard, the iPhone does a great job correcting my mistakes, and it learns my peculiar vocabulary and jargon to its autocompletion dictionary as I go. As with the key clue, this is a great boon to typing accuracy. The Cyrillic based Dvorak hack does neither--there is no autocompletion. The only way to correct your mistakes is to actually correct your mistakes. (It's funny how we whine about things not working well, when they didn't even exist five or ten years ago.)
So while it's nice that there is a Dvorak keyboard hack, in practice, I rarely use it. I thumb type QUERTY on the iPhone, just as I would on any other phone, and touch type Dvorak on the PC's full size keyboard.