While chasing other problems on a 12.04 online upgrade from 8.04 through 10.04, I somehow managed to move the top panel down its own width from the top of the screen. It now obscures a strip of any maximized application.
I found an XML file in one of the hidden configuration directories of my home directory that purports to contain the offsets, sizes, and icons/applets/launchers for both panels. However it still shows a y-offset of 0, although the panel actually appears at what seems to be a y-offset of 36 or so. Rebooting makes no difference; it stays at the offset.
Where else might this information be stored, in xfce 4.8 or xfdesktop configuration files?