I have a treeview control that's first being populated with a list of
items.  Each item has sub nodes, and sub nodes have sub nodes.  It
takes a while to load the whole thing, so I decided the better approach
was just to load the items for the node that's currently selected.

The problem with this approach is the nodes aren't showing the
plus/minus signs next to them until they are populated.  Thus there's
no cue to the user that more is available under them.

What's the best approach to handling this?  I could hack something
together by going through and adding a dummy node under each parent,
then clearing it before populating it with the real thing, but it seems
like there has gotta be a cleaner way.