Multi-player: Invincibility:
In any multi-player level, press Start. You can be shot at, but receive no damage. 
You cannot move, but it is better than dying.

Fighting the Zombiebot King 
In order to beat the Zombiebot King just wear him down using rockets until he 
pauses (doesn't move). Toss a grenade (coring charge) into its mouth. On Normal, 
it takes five grenades to kill him and after the 3rd and 4th grenades there are 
regular zombiebots that swarm the arena

Original Concept FMV sequence:
Successfully complete the game in campaign mode. 

Multi-player General Corrosive:
Successfully complete the game with all secret and speed chips to unlock 
General Corrosive as a playable character in multi-player mode. 

Unlock Concept Movie:
Beat story mode to unlock this feature.