I’ve seen a regular Apple Software Update error – essentially an unknown failure trying to install a new version of iTunes. I finally worked out that this was because I hadn’t opened iTunes between the new install (7.6 in this case) and the previous software update (7.5). On opening iTunes, as I finally did while upgrading my iPod, I saw a “Accept licensing” screen which I guess was preventing the new iTunes installing. So there’s the lesson: open iTunes at least once when you’ve updated it.