Hi,
The Rotation center handle is currently only visible and movable when the Selection tool is active and in Skewing/Rotating mode. This made perfect sense when only rotating and skewing via dragging the rotating and skewing handles utilized the Rotation center handle. Now that the rotating and skewing via the Transform dialog and via the keyboard also utilize the handle it seems to me that the handle should be visible and movable anytime an object or group of objects is selected.
BTW, there seems to be bug in updating the handle when shift-clicking on it to restore it to the center of an object. The XML editor shows the change but the old position is still displayed and used for rotations until the object is unselected and reselected. (Checked with today's SVN.)
Tav