Today that is irrelevant. I challenge you to find a working typewriter in your office that isn't covered in a thick layer of dust. Computers certainly don't care about the speed of the typist. There is nothing to jam and printers today kick out entire pages at a time with only a few moving parts. So why hasn't the Dvorak keyboard layout gained in acceptance? I suspect we are all taught Qwerty in high school or college and once most people can achieve 40-50 words per minute, that is good enough. No point in relearning something you have used for years, or decades, just to get a faster speed, even if that may be 100% faster.
Because there is such a small minority using the Dvorak layout, you can forget ever seeing a phone with a physical keyboard using that layout. However, many phones also have virtual keyboards. The iPhone comes solely with a virtual keyboard. All Windows Mobile devices with touch screens have virtual keyboards. With that, you should be able to install whatever keyboard you want assuming someone has written it. You can run such a keyboard on the iPhone, but only if it is jailbroken. I don't know why Apple would block such an app from the App Store, but you won't find one there.
I feel for Dvorak users. I used a program called FITALY for years on a Windows Mobile phone. It was designed for stylus input and after about 3 months, I could hit 40-45 words per minute with minimal effort. I've seen people hit north of 80 words per minute using just a stylus, so I can understand the Dvorak user's desire for speed. I still have FITALY installed, but now I find myself using the physical keyboard more, which is just fine for two-thumbed operation.
Virtual or physical, phone manufacturers should allow for a Dvorak layout, even if the letters on a real keyboard don't match up. It is just software that the user could easily switch in the settings somewhere to enable it. At the very least, if someone else writes a program, don't block it. (I'm pointing at you Apple.) Give users the chance to personalize their device and make it as efficient as possible. Even if that means that they want to use some obscure odd keyboard layout.