Apparently, some Mac users have had a problem right after Software Update has run on Leopard. It's called the "blue screen of death" for the frozen field of bright blue when restarting the computer. De Prez Jim Hamm found the following tip from "Make Mac Work." http://www.makemacwork.com/startup-fails-with-blue-screen.htm This is most often caused by a corrupt dynamic loader cache, a feature which replaced "application prebinding" in OS X 10.5 and tells the system where to find available shared libraries. Without a roadmap to these libraries the computer can't boot properly.
"If the affected system is running 10.5.6, you can remedy this problem by booting with the 'Shift' key held, triggering a Safe Boot sequence and deleting the damaged cache files. If you're running an earlier version of Leopard, Safe Boot won't remove the caches, so you'll have to boot the machine from a DVD or external drive and remove the files manually from /var/db/dyld/ instead. Once the cache files have been removed the machine should restart normally."
Here, Art Gorski comes to the rescue. He adds the following info to clarify, "Booting with Shift key held down in 10.5.6 automatically deletes these cache files. Restarting the Mac should work fine then." Art continues: See http://applejack.sourceforge.net/ which says, "Using AppleJack you can repair your disk, repair permissions, validate the system's preference files, and get rid of possibly corrupted cache files. The important thing is that you don't need another startup disk with you.
"You can boot your Mac with the Command and S keys held down and enter Single User Mode, which dumps you to a command prompt. There you can type AppleJack to launch the utility. One of the options in this program is to delete cache files. Once done with AppleJack, just type 'reboot.' This method will work with older versions of Mac OS X."
Hopefully, this won't ever happen to you, but it might be good to print up these directions and file them where you can find them.