One of the first things that my son Jimmy asked about when he saw the film previewing the Apple Watch was, "Will I be able to use it?" Jimmy is left-handed.
My reaction was to say that you can probably turn the face 180 degrees, and swap the top and bottom connectors between the Watch and the Watch's strap, and it would work on your right wrist.
This was confirmed by John Gruber, in a tweet from last night:
Left-handers: You can just turn Apple Watch upside down (and swap straps around) and it'll just work.-- John Gruber (@gruber) September 10, 2014