Apple Acknowledges 10.5.6 'Configuring Installation' Issue
Apple released a technote detailing an issue that some users have run into while installing the latest Mac OS X Software Update.
When attempting to update software using Software Update in Mac OS X 10.5, the update process may stop responding while "Configuring installation" is visible in the update window.
Apple describes that this may happen when trying to install a partially downloaded update. The work-around is to remove the partially downloaded update and try again. The incomplete update files may be found in "/Library/Updates" and can be safely deleted before trying Software Update again.
Alternatively, users can download the Combo update (list here) and install it themselves.
It seems that the bug resulting in the partially downloaded file affects Mac OS X 10.5.1 through 10.5.5. Once you successfully installed 10.5.6, the issue seems to be fixed for future updates.