In the second or third mission, where you first see the Korean camp for the Americans full of trailers and RVs, there is a sign that reads "GO WOLVERINES"--a reference to the school's team in the movie Red Dawn.
In the map Farm, go to the tall tower with the yellow ladder. Climb to the top and jump off. There will be a message that says "Press A to deploy parachute" and you can deploy a parachute and survive the fall.
In in Freedom mission, there is a Deus Ex reference. The first line the black-marketeer inside the labor camp says is "You want to talk to El Ray? That's cool!", which is a direct quote from (and spoken with the same inflection as) a body guard for the Rook gang leader in the abandoned Brooklyn Bridge subway station.