Hugh Laurie's got an American accent down pretty well (incidentally, it might do well to point out that there is more than one "American accent.").

But some British actors do seem to have a bit of difficulty getting it down. Also, some of the script writers don't help all that much. All two often an American character in a British show (played by a British actor) will sprinkle his speech with phrases like ". . . ah reckon." The last time I heard an American use the word "reckon," it was John Wayne in an early Western.

Don Firth