var maxV : float = 10.0;
var cSpeed : float =0;

function FixedUpdate()
{
    var rg = rigidbody;
    if (rg.velocity.x > maxV)
    {
        print("baaaaaaaaaa");
        rg.velocity = new Vector3(maxV,         // X
                  rg.velocity.y,    // Y
                  rg.velocity.z     // Z
                 );

    // OR - to avoid having to set the others you should be able to
    //      set individual components of a Vector3 like this;
    rg.velocity.x = maxV;
    }
    cSpeed = Vector3.Magnitude(rg.velocity);
}