As I’ve reported over the years, I’ve never had a Mac OS X update come back to bite me. On the other hand, I can tell you that one friend, a client named Don, encountered an installation glitch when I ran the 10.5.6 update on his brand new MacBook.
Normally, running Software Update on new Apple hardware is a trivial matter. Even if the computer was built but a few weeks before it was shipped to you, you can expect that there will be a slew of updates awaiting download and installation. So just add that to your expected installation experience.
In this particular case, Apple’s Installer app hung on the “Configuring installation” prompt. It turns out this issue appears if the file download process was somehow aborted before it finished. For some reason, Installer launches anyway which is, of course, incorrect behavior. The update should actually resume — or attempt to resume — on the next attempt.
Apple’s published solution is simple. Go to your /Library/Updates folder, and delete the contents. That removes the fragmented file, so you can simply launch Software Update and let it do its thing all over again. Usually it succeeds. Indeed, this partial download glitch is evidently fixed in 10.5.6, but first you have to get there.