According to Kotaku, a conversation was held with a Nintendo representative that confirmed voice chat on the Wii U, or perhaps a lack thereof. The word seems to be that the Wii U will not feature voice chat in any sort of official capacity, rather, that third parties will be responsible for adding it on their own. On top of that, voice chat will not use the built-in GamePad microphone that was previously used in an advertisement for voice chat. Instead, a third party headset peripheral will be required. Welcome to the next generation.
Despite that, there are a few games releasing in the Wii U launch window, including Assassin's Creed III, Call of Duty: Black Ops 2, and Mass Effect, that will include voice chat capabilities. It's rather telling that only the biggest of blockbusters will be launching with the tech. How smaller multiplayer games deal with the hurdle will be proven with time, but we could be facing a realistic possibility that they can't afford developing the tech.
The Wii U is less than a month away from launch, but there are still many details yet to be fully released. It wouldn't be a Nintendo console unless there were some near-required peripherals needed for the best experience, of course.