Cheat mode:
At the main menu, highlight Options Mode and press Down, then press Left or Right until "Option: Cheat Codes" is selected. Press A and then enter one of the following case-sensitive cheat codes:

theworldismine          All locations

greasemonkey            All car abilities

hotwired                All cars in Arcade mode
                        (must be logged on to Xbox Live)

bigbrother              Unlock 'Most Dedicated' stat

lovenotwar              Guns and rockets, press Left
                        stick and White to fire

zoomzoom4               Unlimited nitrous 

howhardcanitbe(0-9)     Change difficulty (0 is easiest)

LAPD Car:
Beat all 5 L.A. Arcade circuit races.

Paris Cop Car:
Beat all 6 Paris Arcade circuit races.

Tokyo Cop Car:
Beat all 7 Tokyo Arcade circuit races.

Rocket Car:
Beat the game 100%.

Thanks to Revolution readers PlanetOne, Matthew, armando villalpando, nirav and MarshaMarsha!