[CentOS] Yum hangs while doing update

Mon Jul 1 17:16:38 UTC 2013
ken <gebser at mousecar.com>

About thirty minutes ago I started an update.  Everything downloaded 
fine.  But when it came to "Running Transaction", it hung:

....
Running rpm_check_debug
Running Transaction Test
Finished Transaction Test
Transaction Test Succeeded
Running Transaction
   Updating       : xulrunner 
     1/18


And there it sits still.

There's plenty of disk space.  The load on the CPU is under 1.0.  There 
is some disk activity, but no way so much as to cause anything to hang 
and no major writing related to yum.  Disk reads are just about null, 
except just now when I grep'ed for something to make sure that the disk 
could be read.  I'm not using even half of the available RAM.

Anyone else run into this problem?