After the native theming that thunderbird3 will have, another issue with GNOME integration has been cleared out. When you used Thunderbird/ Lightning as your calendar the rest of the GNOME desktop would not know about your dates. So for instance the gnome-clock-applet would not show them.
But now there is a add-on available which pushes the data from Thunderbird to EDS(Evolution Data Server), so that they will be displayed in th gnome-clock-applet.
Make sure that you have python-gnome2-desktop and evolution-data-server installed 🙂