Not that I want an American actor to play Bond (I say that, I'm sure there could be some good ones who would still do the accent correctly) but it's funny that an American Bond is right out, but Scots, Welsh, Aussie, Kiwis! Any other English speaking nation is fine!(OK, nobody has ever suggest Canadian I suppose.)
You have to be from a current or former member of the the Commonwealth of Nations, and America isn't one. The original 1926 members are, Australia, Canada, Ireland (1926-1949), New Zealand, South Africa, and the United Kingdom.
(OK, nobody has ever suggest Canadian I suppose.)