Ran into a weird problem with the Snow Leopard install on my Summer 09 Macbook Pro (13 inch). About 5 minutes into the install, the screen went totally black. Thinking that the computer had gone to sleep, I powered down and back up. The machine would load the upgrade from the DVD, but would go black again after displaying the Apple Logo and spinning activity indicator. I figured the machine was hosed, but ran across a very helpful solution on this site:
Turns out the install is continuing, but the backlights on the machine are off! Shine a light on the screen and you will see the installation questions. Great fun finding the mouse cursor on a black screen with a flashlight.
Kudos to this user for figuring this out. Saved me a major heartache.