After I installed 10.6 on a couple of my Macs, I had problems only with my (current generation) Mac Mini that is connected to a Dell television. I could not get any video after rebooting, turning off the TV, etc.
Since I have multiple Macs, I connected to the Mac Mini via Screen Sharing and went to the System Prefs » Display panel, and selected my previous resolution setting. This made the video work right away.
Without a second Mac, I have no idea how someone could fix a problem like this.
[robg adds: This should work for any installation that selects an invalid resolution.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20090829082230579