Some thoughts:
-This feels real to me
-It's likely to be a full-featured, retail game
-Judging the graphics and scope of the game by one piece of artwork is a mistake. Nintendo frequently uses retro Mario graphics to represent the franchise.
-Thus, I would expect more than just 8-bit Mario. All sorts of different tile sets/characters/music etc from the various ages of Mario. Would not rule out 3D Mario completely, or at least the character model.
-This will be Nintendo's big DLC/NFC game. Someone mentioned wanting a Nintendo Maker...well, I have a feeling that's coming, just as additional content to this.
-I suspect this has been in the works for quite a while, as evidenced by the quotes from Miyamoto and Reggie about LBP, and Mario Paint.
-I would go so far as to say this game may be one reason why the Wii U is what it is. Nintendo tends to develop hardware with specific software in mind, but we haven't seen that with Wii U to this point.
Of course, it may not even be real. Just some thoughts and speculation before E3, and isn't that what makes it fun?