statement on the Xbox Live status page reads:
"Hi Xbox members. Are you having trouble connecting to Party Chat on Diablo 3 on Xbox One and Xbox 360? Additionally, Xbox One users, are you experiencing server unavailability issues? Us too, but we're working alongside our partner to get these issues fixed ASAP! We'll keep you posted when we have more information on this issue. In the meantime, thanks for staying patient."
The same group claiming responsibility for DDoS'ing the PlayStation Network and re-routing Sony Online Entertainment President John Smedley's flight earlier today are also implying this to be their work.