I don't know if that helps, but I've deleted my kext-cache at the very beginning and then used OnyX thoroughly, to clean the whole install, and now I have this short boot time with 10.5.5 of about 20sec.

And ACPI works pretty well here for me with 300MB/s... so maybe it varys by the Bios of the boards, and not the chipsets themselves.

Yeah... well, my machine(Acer Aspire 5104wlmi) doesn´t really have any options in the bios...
I guess there might be something I could do with patching the dsdt, but I´m not sure how...

I can add that I have tested using the ioatafamily.kext without any plugins, and I still got the iokitwaitquiet error. While if I just remove the ioatafamily kext then I don´t get the iokitwaitquiet error and the boot time is about 40 seconds shorter, but then only my usb disk works.
I it seems that using the ioatafamily kext is unavoidable if I want my internal disk to boot... at least I haven´t found any other solutions so far.