At the main menu, hold [Left Shift] and press [F3] [F4] [F3] [F3] [F4]. Then, press [F1] to display the cheat menu.
Cheat Mode
While playing a game, press ~ to display the console window, then type "sam_iEnableCheats=1" as a case-sensitive code to enable cheat mode. Press ~ again, then press [Esc], followed by [F1] while in the pause menu. You can now select the cheat options. You can also type "sam_iEnableCheats=2" as a case-sensitive code at the console window to enable Developer mode, or "sam_iEnableCheats=0" as a case-sensitive code to disable cheat mode.
Demo version Cheat Mode
While playing a game, press ~ to display the console window, then type "sam_iEnableCheats=1" as a case-sensitive code to enable cheat mode. Press ~ again, then press [Esc], followed by [F1] while in the pause menu. You can now select the cheat options. Note: This code applies to the demo version of the game only.
Duke Nukem Forever reference
There are several passing remarks about a "Blonde guy taking forever". This a reference to the as-yet-to-be-published game Duke Nukem Forever.
Lava stage: Armor puzzle
Just before you get the mini-gun, there is a door on your right. Open it and there will be armor on the other side. You must to step on the correct letters. The order is: S, E, R, I, O, U, S.
Alternate Cheat Mode...