The problem here seems to have been that KDE (and other things) had been corrupted, presumably by the yum update.
Editing (in fact creating) the file /etc/sysconfig/desktop to contain
made no difference, and executing startkde from the command line came back with "command not found". In the end I did a yum groupinstall of KDE (and other apps like mythtv that had disappeared as well) and everything came back as before, including user settings for all the apps.
The original yum update was quite big (about 400 MB I think) as I hadn't been able to update for several weeks due to the dbus problem, but didn't generate any alert messages. Is there a way for an update like that to corrupt applications this way?