I have seven Macs, including two Intel Macs. On May 11, 2006, Apple released 'Security Update 2006 003' and 'QuickTime 7.1' through Software Update. After installing the update, the Intel Macs would no longer boot. Dead. Nada. I called Apple Care, but they could not fix it.
After some futzing, I managed to fix them this way:
Mac OS X Hints
http://hints.macworld.com/article.php?story=20060518195305900