Here's a quick edit of the smooth list drop applied to a JTree. It doesn't actually handle the drop - it still needs to implement changes to the tree model.
It looks best with a LAF that displays vertical lines, since the lines get stretched as space is made for the insert (OSX doesn't draw connector lines :().
(There was a race condition in the animator in the first posting which would sometimes lock up; thanks to ploiku for the stack trace).
A full implementation for the tree would need to have a few abstract methods defined:
boolean canMove(TreePath src);
boolean canMove(TreePath src, TreePath newParent, int index);
void move(TreePath src, TreePath newParent, int index);
You'd probably want to collapse a non-leaf when dragging it, allow drops onto collapsed nodes, auto-expand collapsed nodes after a delay, and probably a few other things I haven't thought of. Too much for one afternoon, drat.