After a Finder crash, I found that I had two instances of the Desktop in the Save dialog box, but only one in the regular Finder sidebar. To fix the problem, simply delete the preferences file named com.apple.sidebarlists.plist, found in your user's Library/Preferences folder.
This might sound silly, but I went looking in all the wrong places (like com.apple.finder.plist) first.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20060123121706659