Same here, one machine wouldn't boot, took a little work to get it going again, booting in safe mode and then repairing persmissions did it, but I have HEARD that I was lucky and a more involved repair job can be required. This was a much BIGGER update than apple led on, and it touched a number of items, so I would suggest,

repair permissions, BEFORE running the update, find your install DVD before doing the update, and don't do it when you might need the machine quickly thereafter.