According to Penello on Twitter, "We will have an adapter that allows current gaming headsets to work. No, it's an accessory."
Presumably, this is so that gamers with expensive third-party headsets won't have to shell out more money to buy a new one. The basic headset for the Xbox One is $24.99; an adapter is likely less expensive.
Rumorsthat Microsoft will be bundling a headset in Xbox One's after protests from gamers about the lack of one.