Comment 381 for bug 272247

AFAIK, the newer kernels lack the feature of loading custom DSDTs from initrd. Now you have to put your DSDT in kernel at compile time. It means you'll have to recompile your kernel whenever there's an update to it. Currently that's what I do, but on my laptop I use Arch Linux, and I don't have any experience in recompiling kernel in Ubuntu, so I can't provide any specific instructions on how to do that. The basics are here though: http://www.lesswatts.org/projects/acpi/overridingDSDT.php

I believe that for an average user this would be too much of a trouble, and the recompilation takes some time too, so unfortunately for most users it's not a solution.