Cheat: Progressive Scan

During the first loading screen, hold down triangle and X to enable progressive scan. The game will ask if you want to enable it, and after selecting "yes" it will switch to progressive scan and ask if you can see the image. Select "yes" again, and you're good to go.
Note: You must have a television that supports progressive scan for this to work. Otherwise the screen will go blank for 15 seconds and it will eventually switch back to interlaced display.

Cheat: Unlockable Cars

Madden Cruiser:
If you've got a Madden 06 save on your memory card, you can unlock the Madden Cruiser. Boom!

Dominator Assassin
If you've got a Burnout 3 save on your memory card, you can unlock the Dominator Assassin car.

Logitech World Racer
Complete Rank 10 GP Race

Revenge Racer
Finish the Game 100%

Nixon Special
Complete Eastern Bay Challenge Sheet

Hot Rod
Complete Lone Peak Challenge Sheet

Custom Classic
Complete Sunshine Keys Challenge Sheet

EA GT Racer
Complete Central Route Challenge Sheet

Criterion GT Racer
Complete White Mountain Challenge Sheet

Euro Classic LM
Complete Eternal City Challenge Sheet

Low Rider
Complete Angel Valley Challenge Sheet

Etnies Racer
Complete Motor City Challenge Sheet

Black Elite Racer
Get Elite Rank 11