JComboBox is the DragSource with a DragGestureListener registered. When drag operation is initiated with JComboBox, there is no callback from its DragGestureListener - dragGestureRecognized(), therefore, no startDrag() is kicked off.

The test case in /home/tinasu/AWT/DnDTest/Bugs/JComboBox demonstrates the problem.
If the dragsource JComboBox is replaced with AWT List, everything works fine.
The problem is seen on the latest jdk (FCS RC2).