At the menu screen where it says "Press Start," enter these codes, quickly.

Code	Effect
Up, Down, Y, X, X, Y, Down, Up, LB, LB, Start	Unlocks IGN Challenge Room for Xbox 360 (also works on PC version with 360 controller input)
Down, Up, Down, Up, Left, LB, X, LB, X, Y, Start	Unlocks Eurogamer Challenge Room for Xbox 360 (also works on PC version with 360 controller input)