The new date for the US National Championship is a significant change from the initial plans where the US nationals were designated to be played as part of the Week of Nightmares/North American Contintental Championship (June 22nd to 26th, 2011) during Origins.
Quote from the Grand Masquerade website:
The Vampire: The Masquerade 20th Anniversary Convention
Due to the incredible success of The Grand Masquerade 2010, White Wolf invites all World of Darkness fans to join us for The Grand Masquerade 2011, September 15-18 at the legendary Hotel Monteleone in New Orleans. In cooperation with The Camarilla Fan Club, One World by Night, The Garou Nation, The Wrecking Crew and The Elder Kindred Network, the gala affair will celebrate the historic 20th anniversary of Vampire: The Masquerade.
A defining moment and not-to-be-missed event, The Grand Masquerade 2011 brings together the glamour and style from Grand Masquerade 2010 and blends it with the historic intimacy of the very center of the world’s most iconic vampire city, the New Orleans French Quarter.