I had a quick look around and couldn't find this anywhere so I'm posting up some code. This allows you to drag-and-drop the items in the listbox in order to reorder them. A nice UI touch when you want users to have the ability to do this. I'm writing something right now that will use this but thought I'd put this code here in case someone wants to use it.
#include <Constants.au3>
#include <GUIListBox.au3>
#include <WinAPI.au3>
#include <WindowsConstants.au3>
Global $