Comment 22 for bug 147070

I can confirm this too; just happens with Compiz, not with Metacity. Just click anywhere in the desktop and the bottom panel comes from the void.

Also, it's not just the bottom panel that's not visible; I have "session restore" set, and when I login, a Nautilus window I left opened at previous logout is usually not visible. Just click anywhere in the desktop and both the nautilus window (and bottom panel) are finally shown.

To set/unset "session restore" go to System | Preferences | Sessions, tab Session options, check box "Automatically remember running apps when closing session" (I'm translating from Spanish).