In the current version of GNOME there is no way to turn off tooltips on the panel. This is an annoyance when using the Compiz Fusion Window Previews plugin, which shows a thumbnail of a window. The yellow GNOME tooltip stills shows up under the fancy Compiz preview.
The perfect solution would be GNOME having an option to turn off tooltips for window list items. But I found a way to solve the problem using Compiz Fusion on the Compiz Fusion blog.
Open General Options in the CompizConfig Settings Manager. Go to the Opacity Settings tab and add an item to the window opacities list. You can match only the GNOME panel tooltips with this code:
(name=gnome-panel & type=tooltip)
Then set the opacity to zero and the tooltips will be totally invisible. Tooltips for programs running in the system tray still show up like they should.
A lot of cool stuff is possible with the advanced settings that Compiz offers. I made my panel semi-transparent, which works better than the GNOME option to do so because it affects the menus also. Here are the settings I used:
opacity windows: (name=gnome-panel)