Wheel colliders aren't easy things to control, when making a bike or motor you'll have to do some tricks to make it stable (not falling) as the wheel collider is infinitely thin. Below I wrote a simple motor script (that doesn't solve the falling over problem, but set y of centerOfMass property to negative values does help).

Because the wheelcolliders shouldn't rotate with the wheels, you need to set up the motor as follows for the script, the motor has two wheel attach gameobjects (frontwheelattach and backwheelattach), you must add the wheelcolliders to these nodes (and attach these to the script properties front/backWheelCollider). The attach nodes must have the actual wheels attached, and these should be connected to the script front/backWheelTransform.

If you need a demo project to see how to setup the script I could make something quick for that as well, just let me know.

// Jaap Kreijkamp
// Ctrl-J (http://ctrl-j.com.au/)
// Demo motor script, do with it whatever you like...

var frontWheelCollider : WheelCollider;
var backWheelCollider : WheelCollider;

var frontWheelTransform : Transform;
var backWheelTransform : Transform;

var motorPower : float = 50;
var maxSteerAngle : float = 10;
var centerOfMass : Vector3;

private var backWheelRotation : float = 0;
private var frontWheelRotation : float = 0;


function Start() {
    rigidbody.centerOfMass = centerOfMass;  
}

function UpdateWheelHeight(wheelTransform : Transform, collider : WheelCollider) {
    var localPosition : Vector3 = wheelTransform.localPosition;
    var hit : WheelHit;

    // see if we have contact with ground   
    if (collider.GetGroundHit(hit)) {
        // calculate the distance between current wheel position and hit point, correct
        // wheel position
        localPosition.y -= Vector3.Dot(wheelTransform.position - hit.point, transform.up) - collider.radius;
    }
    else {
        // no contact with ground, just extend wheel position with suspension distance
        localPosition = -Vector3.up * collider.suspensionDistance;
    }
    // actually update the position
    wheelTransform.localPosition = localPosition;
}

function Update() {
    var deltaTime : float = Time.deltaTime;
    var accel = Input.GetAxis("Vertical");
    var steer = Input.GetAxis("Horizontal") * maxSteerAngle;

    // set power
    backWheelCollider.motorTorque = accel * motorPower;

    // set steering
    frontWheelCollider.steerAngle = steer;

    // calculate the rotation of the wheels
    frontWheelRotation = Mathf.Repeat(frontWheelRotation + deltaTime * frontWheelCollider.rpm * 360.0 / 60.0, 360.0);
    backWheelRotation = Mathf.Repeat(backWheelRotation + deltaTime * backWheelCollider.rpm * 360.0 / 60.0, 360.0);

    // set the rotation of the wheels
    frontWheelTransform.localRotation = Quaternion.Euler(frontWheelRotation, steer, 0.0);
    backWheelTransform.localRotation = Quaternion.Euler(backWheelRotation, 0, 0.0);

    // now some more difficult stuff: suspension, we have to manually move the wheels up and down
    // depending on the point of impact. As we have to do it twice (two wheels) we put it in a separate function
    UpdateWheelHeight(frontWheelTransform, frontWheelCollider);
    UpdateWheelHeight(backWheelTransform, backWheelCollider);
}

I fixed the problem by myself (I'm so proud now :D) the reason of the wheels sticking in the middle of the vehicle is that in our UpdateeWheelHeight function there is this last line of code to be replaced: wheelTransform.localPosition = localPosition; with this: wheelTransform.localPosition.y = localPosition.y;

it's just that we have to control JUST and ONLY y axis of our wheels, not all 3 of them. so when the vehicle was in the air the code was repositioning the object on all of the axis - moving the whole wheel to (0,-suspensionDistance,0) in local coordinates.

PS: I found another minor mistake here: localPosition = -Vector3.up * collider.suspensionDistance; it would be a better idea to subtract a radius of wheel from suspension distance: localPosition = -Vector3.up * (collider.suspensionDistance - collider.radius);

Kevin: GetGroundHit is intended to produce false when the colider is in the air. that's why we are using the if statement :).