When upgrading to Snow Leopard, I encountered an apparent bug. I started the upgrade process and walked away for a couple of hours. When I came back, the screen was black and the machine appeared unresponsive.
I assumed that the upgrade failed. Thanks to the backup program SuperDuper, I booted off of the backup, and started to restore the computer from the backup.
In the mean time, I searched in Google for an answer. I learned that the upgrade was successful, but the screen blanker had activated, and would not turn off. I let SuperDuper restore the computer to try again.
How to Avoid the Problem
Before upgrading to Snow Leopard, go to System Preferences, and click on the Energy Saver. Make a note about the position of the Display sleep slider.
To avoid the problem, move the Display sleep slider to Never. If it warns you that this will take up more energy, ignore the warning. This is a temporary action.
Upgrade the operating system per the Snow Leopard instructions. You should not encounter a black screen if you walk away. Go to the System Preferences, click on Energy Saver and restore the Display sleep setting.