Many times we screw our gnome desktop panels accidentally. I found a very easy solution to revert them.
Just open terminal and type.

gconftool-2 --shutdown
gconftool  --recursive-unset /apps/panel
rm -rf ~/.gconf/apps/panel
pkill gnome-panel

But remember this is not the case when gnome panels get scrambled automatically (Which seems to be a consistent case in Ubutnu 10.04). You can simple refresh you gnome panels by doing.

killall gnome-panel