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.