As an addition to this, it would be great if moving pivots could be toggled off altogether, so that the pivot always stays at 0,0,0. The current model of grabbing the surface point under the mouse as the pivot makes simply spinning around a model quite difficult as with each rotation, the pivot will come closer to you as will the model.
An immovable pivot is also nice in so far as you don't have to care for where your cursor is when tumbling around. Viewport navigation should be something you don't have to think about.