Proportional Editing - Limit to selected? This seems an obviously desirable option for proportional editing: to be able to transform only selected elements, using, say, the active element as the center of falloff. Is there a Blender way to achieve this reasonably quickly? 

I know it can be done by hiding the elements you wish to remain untransformed, but often you want to see them during the transformation.
One workaround is to select the vertices that you want to leave unaffected and press H to hide them. Proportional editing will only have effect on the visible geometry.