Official EU Playstation blog writes:
Hello, everyone! Ash Monif from Subatomic Studios here to reveal super secret PSP minis cheat codes for Fieldrunners!
Super secret cheat #1 – Unlock All Maps and Fast Forward
Now you can instantly play any map and mode with fast forward!
1. Navigate to the Credits screen.
2. Enter the following button sequence: Down, Down, Up, Up, Right, Left, Right, Left, Circle button, X button.
3. The "Charge" sound effect will play indicating the cheat was unlocked!