Unfortunately I was doing the update at work so I didn't have the VGA-dongle-thingy to confirm. But when I started the install, I had the screen down to minimum brightness, which is how I almost always have it.
When running the installer, it reboots the machine and then proceeds with the usual install stuff - asking questions, and the like. But after it had rebooted and started to run the installer (ie, after the Happy MAc face had come up, and it was showing all the usual signs of having booted) the screen went dark. Couldn't tell if it was just the backlight or if the screen was indeed blank, or switched off.
Anyway. Eventually managed to convince it to boot and not try and do the install. It booted happily into the existing 10.0.4 system, where I set the brightness to maximum. Ran the installer, and lo and behold it rebooted to do the install, and the screen was visible. Who knows, maybe the installer had a different way of doing screen brightness and rounded it down to zero. Or something.
Tis installed now, and quite groovy, much faster like everyone's been saying. Thank goodness. Except the command line (Terminal) seems to take longer to launch. So be it.
Have installed the dev tools also, which will probably keep me entertained for at least part of today.