[Yum] --download-only doesn't download

[Jason S. Friedman]

I'm running Fedora Core 2.

$ yum check-update
yields a list of a few dozen packages that could be updated.

However,
$ yum --download-only update
just downloads the headers.  Nothing has been added
(recently) to /var/cache/yum/updates-released/packages.

It's especially odd because a couple of weeks ago I used
'check-update' followed by '--download-only update' to
successfully download a bunch of rpms.