I got the same message, so I downloaded the update directly from Apple's website. After I finished installing it, I hit Restart in the Installer, and nothing happen (except the button grayed out and I couldn't click it again). I quit all of the open programs, and left it overnight (at work), but it was still sitting there in the morning, so I had to shut it down hard. It's working for now and says that I'm running 10.5.7, but I'm not convinced that it worked right. I'm gonna be keeping a close eye on this system for a while.
jW