Ok, I'm really not going to read through all of this to see if these ideas have already been put out there, so forgive me if I'm beating a dead horse. I just found out about this, and want to put my thoughts down somewhere.
I think (and hope, and pray) that this is fake. Along with all the other stuff I've seen mentioned so far (roster discrepancies mostly), I have a couple of points. First, apparently
ESRB staff don't play games they rate during the rating process. So, that seems to debunk a lot of it. Second - and this one I like a lot -
Ness's shirt in the leak is inconsistent with past iterations. It would be strange for Nintendo to pay such close attention to that one detail for the past decade, then just forget about it now, when details can be so much more... well, detailed.
Finally, I noticed some wierd things about some of the images themselves, specifically about the ones that don't have any break between the top and bottom screens. This could be from a direct capture device, but if that is the case, why are they all skewed at random angles? A direct capture device would be like taking a screenshot. It looks to me like the leaker was trying to make it look more "leaky" by skewing the images a bit.
Also, if the leaker brought a direct capture device with him, don't you think he'd have been able to bring a better camera as well? Any smartphone takes way better pictures than that, and I find it unlikely that someone working in the games industry would own an old non-smart-phone.
Anyways, those are my thoughts. I hope at least some of them are original. Peace!