For various reasons I use Thunderbird as my calendaring app. One of Gnome-shells most obvious features is it’s clock applet front and centre on the screen, with it’s built in calendar. It pulls this information from Evolution.
This config change will get the applet to launch Thunderbird (or any other app), but it will still not populate the calendar with entries from Thunderbird. But this single line, will mean that Thunderbird launches when you click on the in-built calendar.
$ gsettings set org.gnome.desktop.default-applications.office.calendar exec 'thunderbird'
To populate the calendar widget there is an extension for Thunderbird called: Evolution Mirror which should populate the evolution db with your Thunderbird data. You will need to install a couple of additional packages to make it work, in F16:
$ sudo yum install gnome-python2-extras gnome-python2-evolution