Space War

FWIW, if you change the tilt mechanism line to

rigidbody.rotation = Quaternion.Euler ( 0.0f, rigidbody.velocity.x * (tilt / 4.0f), rigidbody.velocity.x * -tilt);

your ship will also tilt slightly left/right, which gives better feel to the game and allow shooting slightly left and right. (This is presuming that this is based on the 'Space Shooter' tutorial.)