Why does my mail client/web browser not appear in GNOME?

[Bastien Nocera]

Or why is my dingus click not working?

Your web browsers and mail clients need to handle
"x-scheme-handler/http"[1] and "x-scheme-handler/mailto" respectively to
be listed in the GNOME default applications in the new control-center.

Once our default applications are setup, I'll make the necessary changes
in shared-mime-info for the applications to be picked up.

See:
http://www.hadess.net/2010/10/new-control-center-and-you.html
and
http://standards.freedesktop.org/shared-mime-info-spec/shared-mime-info-spec-latest.html#id2869854

for details.

[1]: And probably x-scheme-handler/https