The fact that this is coming out in August means NSMB Wii U probably isn't going to come out until sometime in 2013. Since the Wii U isn't going to come out until November, there's no way in hell Nintendo is going to release two 2D Mario games within 3 months of each other. Mario Galaxy 2 was done by the end of 2009 but Nintendo delayed its release over 6 months because they didn't want it to take away any attention for NSMB Wii's during the 2009 holiday season. Considering gameplay wise, Galaxy 2 was very different from NSMB, and yet Nintendo delayed it 6 months, I wouldn't expect NSMB Wii U to come out until at least mid 2013 now.
I mean really, just look at how the holiday season would go this year if Nintendo released both games. One 2D Mario game for $40 on a $170 system, while another ones $50 on a $300 system. Yeah I'm pretty sure Nintendo knows which game the majority of the Mario audience would by this holiday and sense the Wii U is launching this November and will really need the holiday sales to give it a great launch, using a NSMB just isn't an option anymore since the 3DS will have just gotten a similar game and take away most attention a Wii U version would originally have gotten during the same time.