Probably fake considering nintendo said they wanted to release and original title first, BUT I can't think of a better time in history to announce a game like MM than the supposed end of the world, and Nintendo flip-flop on what they say all the time so it really is anyone's guess. Nintendo can be so unreliable with their news sometimes that i think it could be entirely possible that the 'original' Zelda game for 3DS will be the sequel to ALttP that they're so desperate to make.
It's definitely fake. I doubt we'd get leaks of an MM3D, especially since Nintendo said they're looking to release an original title before anotehr remake anyway. Besides, MM has to do with "end times" just like the whole 12/21/12 thing does. :I
It seems pretty legit, but I'm still a little bit suspicious. After all, Nintendo did say that they were planning on releasing an original title first. Although it wouldn't be the first time they've changed their mind about something...
If it is just a hoax I'll be pretty disappointed, but at the same time I wouldn't be all that surprised.