Comment 7 for bug 544962

I'm currently using Ambiance, the default theme in Lucid, and I've noticed the following behaviour.

When I get the white line, I can make it go away by going to System > Preferences > Appearance, change to another theme (e.g. Clearlooks), and then change back. When the panel goes back to Ambiance, it does so without the white line.

But if, instead, I simply try to do
$ killall gnome-panel
the white line does not go away when the panel reloads.

I imagine #6 is faster than my method if someone just wants a quick workaround to make the white line to go away, but hopefully the information I've provided might be of some use in finding a solution.