I think that most of you know the situation: You want to install OS X on an old iMac G3. If you don't update its firmware before the installation, your iMac won't display anything on the screen any more. I had this issue in the past when I didn't know about that requirement. On some models, you get at least a very(!) dark screen, but most will just stay completeley black. Once I even had one which was dark pink. In the past, I used different tricks to revive the dead iMac:
Not having installed OS X on such an ancient machine for a long time, I forgot about the firmware update requirement, and booted off the OS X Installer CD. I think you can guess what happened then. Firmware too old, no screen. So I went through the steps above and at the point of creating a bootable system with a VNC Server on it (like I have done succesfully in the past), I learned something even easier.
Start up the screen-dead iMac, let it boot into OS 9, and wait until the Finder has loaded -- you can guess that pretty well by the sound of the HD. Then push the On/Off button to put the iMac to sleep. When the green LED turns to a blinking orange, your iMac is asleep. Then wake it up with a keystroke on your keyboard and (presto!) you have your screen back!
Be aware that the display only will work until your next reboot/shutdown, so now would be a good time to run the Firmware Updater! After the update, the iMac will boot once more with a dead screen (as stated above), but after another shutdown, it will be alive again from then on. You can install OS X without any problems now. I know that the problem should be well known, but I haven't seen this simple solution anywhere on the net, so I decided to share my experience.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20050218164736471