Server error!
The server encountered an internal error and was unable to complete your request. Either the server is overloaded or there was an error in a CGI script.
If you think this is a server error, please contact the webmaster.
Error 500

Turned out that the .htaccess file in the root of the WordPress install had some gibberish in it.  It looks like this gibberish replaced some of the actual contents of the file.  I don’t know how it got there, but fixing the .htaccess file solved the problem.  So, that’s something to check if you are having this problem as well!

In other words, if you have a server running Mac OS X Server 10.5, you can’t use it to serve updates to client machines running Mac OS X 10.6, unless you upgrade the server to 10.6 as well.  This limitation seems a little silly, surely Apple could push an update for Mac OS X Server 10.5 that told it where to look for 10.6 updates.  The update service itself doesn’t seem to have really changed — you can tell your 10.6 clients to check your 10.5 server for updates, and it will work fine, you’ll just miss out on updates specific to Mac OS X 10.6.

…or is there a way around this limitation?

Here is how to configure Mac OS X Server 10.5 to serve updates to 10.6 clients.  A disclaimer: I am sure this method is not supported (or maybe even frowned upon) by Apple.  But, it’s working for me.  Also, I am assuming that you are an experienced OS X admin, and I am not responsible if your server blows up or otherwise exhibits unexpected behavior as a result of you following these directions.

• Stop Software Update Services on the server.
• Create a file with the following content and save it to /usr/share/swupd/html/content/meta/mirror-config-1.plist:
  

  <?xml version="1.0" encoding="UTF-8"?>
  <!DOCTYPE plist PUBLIC "-//Apple Computer//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">

  PrimaryCatalog

  http://swscan.apple.com/content/catalogs/index.sucatalog

  CatalogsList

  http://swscan.apple.com/content/catalogs/index.sucatalog

  http://swscan.apple.com/content/catalogs/others/index-leopard.merged-1.sucatalog

  http://swscan.apple.com/content/catalogs/others/index-leopard-snowleopard.merged-1.sucatalog

• Change the metaIndexURL in /etc/swupd/swupd.plist as follows:
  

  metaIndexURL

  http://myserver.com:8088/content/meta/mirror-config-1.plist

• Start Software Update Services and wait for it to download the catalog files.
• Generate symbolic links to expose the new catalogs on the local web server:
  

  cd /usr/share/swupd/html
  ln -s /usr/share/swupd/html/content/catalogs/index.sucatalog . # this one is probably already present
  ln -s /usr/share/swupd/html/content/catalogs/others/index-leopard.merged-1.sucatalog .
  ln -s /usr/share/swupd/html/content/catalogs/others/index-leopard-snowleopard.merged-1.sucatalog .

• Restart Software Update Services again, just to be sure.  (After a while, you should notice that it has downloaded some 10.6 updates.)
• Change the URL that the 10.6 clients use to check for updates.  Normally, you would use this command:
  

  defaults write /Library/Preferences/com.apple.SoftwareUpdate CatalogURL http://myserver.com:8088/

  But now, we’ll use this:

  defaults write /Library/Preferences/com.apple.SoftwareUpdate CatalogURL http://myserver.com:8088/index-leopard-snowleopard.merged-1.sucatalog

• Now, the 10.6 clients should be able to get their updates from your 10.5 server. 

Now, this wasn’t my idea, I found it in response to a query someone made at Apple’s support web site, back when Mac OS X 10.6 was still pretty new.  I am posting it here in hopes that it will help someone else down the line.