All missions:
Press X, L2, Triangle, R2, Select at the title screen to unlock all missions.

All Special Features:
Press L1, L2, R1, R2, X, Select at the title screen to unlock all specials features.

Invincibility:
Begin a mission, then pause game play. Press L1, R2, L2, R1, Select to become invincible. Your first soldier will be invincible, but the remaining members of your platoon can still be harmed.

Hint: Medals:
Purple Heart: Get wounded or killed in action.
Bronze Star: Get 15 kills
Silver Star: Get 20 kills
Distinguished Service Cross: Get 25 kills
Congressional Medal Of Honor: Get 30 kills