When I selected "Software" and within that chose the menu item to check
for updates, the process worked for a while and then apparently made no
more progress but ate 100% of the CPU.
It is kind of scary interrupting an update, but after about half an hour
of no progress, I decided that there was no other way.
Do this in a superuser shell instead. It works and gives better feedback:
yum update
See <
https://bugzilla.redhat.com/show_bug.cgi?id=922697>
PS. I consider software: check for updates less convenient than the
pre-F18 "sofware updates".