Microsoft has already announced that the Twitch.tv app will be available for Xbox One U.S. users for the launch of the console on Friday. However, the app won't be able to offer Xbox One owners a way to broadcast gameplay footage from their console to the service until sometime in the first half of 2014.
In a post on the Xbox Wire site, Microsoft said:
We know the ability to instantly broadcast gameplay is something the gaming community is excited about, and we are too. We are working to ensure the initial Twitch on Xbox One broadcasting experience meets the expectations of the Twitch community, so while this feature won’t be available right away, we’ll let you know as soon as it is ready.
The Twitch.tv app made for the PS4 is able to broadcast gameplay footage from the console to the service, and the PS4 also supports Ustream for the same features.
Microsoft's blog post does state that the Twitch.tv Xbox One app will allow users to view livestream videos from the tens of thousands of channels on the service, including presumably the channels that feature PS4 gameplay. Like the Machinima app, the Twitch.tv app also has its own list of media specific achievements that can be unlocked by users.
Source: Xbox Wire