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On most PC keyboards the keys have in each row have a different shape. The tops of the keycaps are at a different angle. This provides a slight front-to-back curvature of the top surface of the keys. (You can see this if you peer at the keys from the side).
If you go moving keys between rows, the tops of the keys will not line up... the keyboard will look strange and feel "bumpy" and uneven. Not good for touch typing.
Solutions:
A flat keyboard - a few manufacturers produce keyboards where all the keycaps are the same shape. Apple and Sun keyboards are like this, and I think most Logitech keyboards too... But be careful of the new Apple aluminium keyboard -- it may be harder to remove the keycaps safely.
Also, you are best starting with a US QWERTY keyboard. Other national layouts have some different keys, and you won't be able to get standard Dvorak by re-arranging.