You know... as much as I like Dark Link... I think it would just be too much for this particular game... I know there's supposed to be a lot in it, but if they did add him in there... I don't know, it'd just feel really gimmicky to me.
I'd like to fight Dark Link with the whole motion-controlled thing, as long as he doesn't turn into a character, and just stays an enemy, that'd be really cool (as long as you can't just spaz in a corner to beat him)
So long as Dark Link didn't mimic every one of your movements, and we didn't have to outsmart him overmuch, I wouldn't mind him being in Skyward Sword. But I have him in mind as more fitting for another game, as the main villain, since SS's archenemy is already revealed.
I would love for him to appear as a major boss a la AoL, however I doubt that will happen seeing as how we've seen (or some of us, depending on who is into spoilers) two enemies who are particularly good blade-wielders.