Good news, I think. We've been able to switch off the Nvidia graphics adapter after booting Linux, which means it's possible now to boot directly into Linux and have X11 working with DRI and compositing enabled without the trip through booting into XP first.
It's quite a crude method, though. The hack with acpi_evaluate_object() I mentioned in my previous post worked, I just call the "_DSM" ACPI method of the built in Intel chipset graphics card and instruct it to call the "_OFF" ACPI method of the Nvidia card. I've also found out how to switch the Speed and Stamina LEDs so that you have some visual feedback of what's happening. You cannot see any difference in lcpci, which means that the card is still registered to the bus, apparently it's just powered down. I have no idea currently how to detach it entirely from the bus to reproduce exactly the configuration after having booted XP, maybe through PCI hotplugging? That being said, I have also no idea yet if the card is really powered down entirely.
My wife Eva has uploaded our experimental sony-laptop.c, grab it here. I don't make any guarantees about not damaging your precious toy if you dare trying it.
We're quite pleased with the current status however. It means that Eva can now shut down the laptop without having to make the stupid WinXP detour before using Linux again.
Her detailed log of installing openSUSE 11.1 on the Z21 can be found here.