Set the Xbox 360's system date to Veteran's Day (November 11). Go to "Custom Games" and the Valhalla map. Take a Banshee to the middle of the map.
Go to the opposite side of the crashed Pelican. On the huge dam-like wall, there should be a message directly in front of the Brute Shot stating that "Kilroy was here" in a white font.
Also, if you face the wall, turn right, and walk towards the rock in front of that base, you will see the same message (with the same picture) under the grass.
Enjoy!
