Last i heard that while DVORAK are supposed to allow you to type faster, tests show that it doesn't provide any advantage over QWERTY in speed (ie typists get retaught DVORAK compared to typists who spend the same time working on their QWERTY show negligible difference).
Short version, no or negligable improvement, massive cost to change over.
I don't believe that for a second. All the typing records are set on Dvorak keyboards: why might that be?

