The Diancie Event is now live in the United States. You can obtain this Pokemon by going into Gamestop and obtaining a Serial Code.
The Diancie is at Level 50, and knows the moves Diamond Storm, Reflect, Return, and Moonblast. It will be holding a Normal Gem, and will have the Wishing Ribbon attached to it.
Diancie cannot be found during Gameplay; this is the only known legit way to obtain a US Diancie.
This Diancie, when traded/transferred to Pokemon Omega Ruby / Alpha Sapphire, will allow you to obtain Diancite and Mega Evolve Diancie.