A recent post on Kotaku reveals that in-game voice chat will only be supported on some selected games such as Assassin’s Creed III, Call of Duty Black Ops II and Mass Effect 3, all games with significant online multiplayer modes. And if you do want to use the in-game chat, you’ll need to plug in another headset to your controller, the Gamepad’s own mic is apparently reserved only for OS chat.
Players can use licensed stereo headsets from Turtle Beach or Mad Catz (Tritton), as no official Nintendo mic has been revealed. It seems a bit counterintuitive to need two mics to do the same function, one would think the Gamepad mic could pull double duty. Furthermore, the Wii U Pro Pad doesn’t have a headset jack, so if you want to voice-chat and use the Pro Pad you’ll have to get a Gamepad, a headset, connect them together and have the Gamepad sitting next to you while you play.
Lastly, it seems that there are no wireless headsets for Wii U at the moment, however a statement by Turtle Beach reveals that the reason there are currently only wired headsets available is that they wanted to have something ready at the time of launch and that the wireless sets are on the way.