I was more skeptical last night, but today I'm leaning toward believing the leak. As was said, apparently the leaker would have had to come up with a number of professional-looking fake renders, along with a pretty believable update to the Duck Hunt Dog design.
Also, I find the existence of Dr. Mario and Dark Pit to actually lend credence to the leak. There was always going to be something questionable about the final roster, and reviving Dr. Mario as a full slot and adding a third Kid Icarus character seems right in line with Wolf being the final character in Brawl.
Plus Bowser Jr. does make sense as a character, I actually would have put money on him getting in ahead of Rosalina. I would have assumed as a lighter, speedier Bowser clone though . . .