Zelda needs the Wii U. The Wii U needs Zelda. Nintendo knows this, and that's why I think the next Zelda will be released Q4 of 2014. Of course, I have some evidence to back this up.
First, I have a quote from the man, the myth, the legend, Shigeru Miyamoto.
“We’re also working on a new Wii U Zelda game. Work on that is progressing fairly concretely. In fact, we were thinking about showing it here at E3 but decided to wait just a little bit longer before sharing it with everyone.”
Now, if the game was in a showable state, it must be up and running on the system correct? Development periods for Zelda titles are usually 3-4 years. We got Wind Waker in 2003, and Twilight Princess about 3 years later. Hell, the gap between Ocarina of Time and Majoras Mask was two years, and the gap between Majora's Mask and Wind Waker was 3, so we have a bit of a pattern going on. Take note that Skyward Sword was released way back in 2011. When development started for Zelda U is unknown, but I'm assuming it was right after Skyward Sword, maybe even during.
Now remember back in January when Nintendo held that legendary direct? They said that Wind Waker HD was meant to tide us over for the next Zelda title, now naturally, a game as big as Wind Waker HD would usually take 100 hours to beat, if you completed it in both regular mode and hero mode. Naturally, the game won't last that long if you play it avidly. HOWEVER, if you play it casually and alternate between other games and etc, it would take 9-10 months to beat, which leads me to think that during that time Nintendo will reveal Zelda U and give us a solid release date.
We also have the release of A Link Between Worlds, the first original Zelda title for 3DS. This might serve as an experiment for Zelda U. I say this because it allows you to do dungeons in any order, and how this is recieved might affect how development continues on the Wii U.
This is all speculation, but I think that the game will come out sooner than we expect. Nintendo knows the Wii U needs a new Zelda, and they have hired a ton of employees to get the game out in a timely manner.