Opening up the console anytime during gameplay and entering the word "haiku" will freeze the game and display a random haiku on the screen.
There is a hidden message on the upper skybox texture in the V-22 osprey g-man sequence. It reads "HACK HACK HACK ALL DAY LONG. HACK HACK HACK WHILE I SING THIS SONG". This is a reference to the Adam Sandler song "The Beating Of A High School Janitor".