Something along these lines should do it

var maxVelocity : float = 10.0; //set in the inspector

function Update()
{
    var velocity = rigidbody.velocity;
    if (velocity == Vector3.zero) return;

    var magnitude = velocity.magnitude;
    if (magnitude > maxVelocity)
    {
        velocity *= (maxVelocity / magnitude);
        rigidbody.velocity = velocity;
    }
}

For rigidbpdy, I use FixedUpdate http://unity3d.com/support/documentation/ScriptReference/MonoBehaviour.FixedUpdate.html

var velocityMax : float;    //say 10 @ the inspector
var speedRate: float;       //say 2 @ the inspector
function FixedUpdate()
{
    var vMotion = Input.GetAxis ("Vertical") * speedRate;
    var velocity = rigidbody.velocity;
    var vMagnitude = velocity.magnitude;
    if(vMagnitude < velocityMax)
    {
    rigidbody.velocity += vMotion;
    }
}

In this case, more velocity will be added only if the velocity is less than the limit (velocityMax)

maybe you could increase the drag property on inclined downward slopes.. it is not the best solution but it worked for me.

See this question: http://answers.unity3d.com/questions/2502/limiting-rigidbody-velocity