There will be a default selected in gnome-session, that can be changed by user action. Without a session-wide default (any session manager can set one, of course) a random choice (possibly alphabetical) is used.
It seems the panel to set the preferred applications has been removed from future versions of the control center on purpose of the GNOME developers:
http://osdir.com/ml/fedora-development/2011-02/msg00116.html - Fabian