I use Dvorak as my main typing layout. I own a Motorola Xoom tablet, for which I bought an external bluetooth keyboard. I couldn't find how to use Dvorak on the physical keyboard though; I tried some of the Dvorak-capable keyboard apps in the App Store (Hacker's Keyboard, Honso Multiling, XenionSH Dvorak Keyboard), but none of these seemed to help (ASK has an open issue for this: http://code.google.com/p/softkeyboard/issues/detail?id=181)
Has anyone managed to do something similar (using either an external BT keyboard or a phone with an inbuilt physical keyboard.)