[CentOS] yum update -> [Errno 4] Socket Error: timed out

Trying to update a second CentOS box, I'm getting this error repeatedly:

[Errno 4] Socket Error: timed out

I'm getting this on every mirror and have gone through the list of
mirrors more than a dozen times.

Oddly, the RPMs I'm trying to upgrade I upgraded just yesterday without
a problem on another machine on the same LAN with no problems
whatsoever.  I can ping mirrors fine.

There were a spate of these errors back in 2006.  The fix for many was
to add this line to yum.conf:

timeout=300

So I did that on the machine where yum is having the problem, but the
same errors are returned.

Anyone else seeing this?  Anyone know what the problem is?