If you have Firefox and use Xmarks you could sort them alphabetically in Firefox and the changes would carry over.
If you just want to use Safari: bookmarks menu - edit bookmarks - drag the folder you want to alphabetize to the desktop - sort by name - delete the folder in safari - drag the folder from the desktop back into safari.
Hope the 2nd makes sense. Its an annoying work around. It would be nice if there was a menu to sort like in Firefox.