When running Sherlock in 10.3, I found that all the standard Apple channels were missing. This may have been from installing some of my own custom channels when running 10.2. I found an Apple Discussions tip which worked for me. You just have to delete your Sherlock preferences. Where "~" means "your home directory," delete ~/Library -> Preferences -> com.apple.sherlock.plist and the ~/Library -> Preferences -> Sherlock folder to see Apple's channels again.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20031102162120397