jeffgreco's reply addresses it, but just to clarify, I wasn't thinking about mirrored dvorak keyboards or stuff I don't even know exists. More things like Canadian qwerty or japanese JIS keyboards, that are main stream in each country but will need special effort for Microsoft to manufacture. The ipad avoids the problem entirerly.
Look, I'm not cheerleading Microsoft here: I'm a Linux desktop developer. If anything, I'm saying these things with a tinge of resignation (not about Microsoft specifically, but the way the market works - though I prefer to accept it as reality, and see it as a challenge).
Now, one thing that might be true: A sanctioned solution leaves less room for accessory makers, so that Siberian Dvorak keyboard may be more likely to be available for the iPad than the Surface, leaving the iPad as the obvious choice for Siberian Dvorak users.