I had the need to update a server that was acting as a Software Update Server. I knew that the machine had the update on it, but was unsure of how to get it to see the locally-hosted update. If you open Terminal and issue this command...
open /usr/share/swupd/html/index.sucatalog
...then Software Update will launch, looking at the localhost -- no need to re-download the update from Apple.
Mac OS X Hints
http://hints.macworld.com/article.php?story=2009100210290464