First news that it was found was posted in the New Mombasa forum over on bungie.net. I posted a notice of it over on HBO, but to summarise:
During a medal ceremony in the opening cinematics of the game, the Sgt. Johnson character is presented with a medal by Admiral Lord Hood (voiced by Ron Perlman) which is the highest reward for "a soldier of the United Earth Space Corps..."
Until then, the governing body in the Haloverse was the UNSC, the United Nations Space Command. The UESC reference here has to be another nod to Marathon.
-- Steve's ashamed to admit that he didn't spot it first, despite playing the game nine times so far.