You say you had the same problem with Arch Linux and solved it by adding the setpci command to /etc/rc.local. How did you do that if your screen was black?
Anyway, assuming the modifications given in the Arch Linux forums would work, the method of applying them in SL would basically be the same. You quoted two modifications: a change to the local initscript and a change to the kernel boot line. For the latter the easiest thing to do would be to edit /boot/grub/grub.cfg and add acpi_osi=Linux to the end of the kernel boot line. For the former, if your SL installation has a version of OpenRC lower than 0.8.0 then you would have to edit /etc/conf.d/local and add that setpci command to it, whereas if you have OpenRC 0.8.0 or greater installed then /etc/conf.d/local would not be used and instead you would have to create a file /etc/local.d/setpci.start with that setpci command in it and make it executable: chmod +x /etc/local.d/setpci.start.
But if your screen goes black when you boot an Arch Linux LiveDVD, I don't understand how you managed to apply to it the modifications you quoted. And the same comment applies for a SL 5.5 LiveDVD. You did mention that you didn't have the problem with SL 5.4, so you could install SL 5.4, perform all the above edits (including the creation of /etc/local.d/setpci.start even though it will not be used by SL 5.4 and 5.5 as installed from a LiveDVD (OpenRC version 0.6.8 is on SL 5.5 LiveDVD)) so that when you upgrade using Equo or Sulfur you don't run into the problem.
