In case you don't wanna build gnome-panel from sources, you can simple patch it in binary!

Here is patch for Ubuntu 11.04 natty:
00049D9E: 08 7F
00049FD0: 08 7F
0004A252: 08 7F
Disassembling, patching, backuping, replacing and restarting gnome-panel takes less then 5 minutes.
That is how i did it.