Len Brown of Intel's Open-Source Technology Center spoke about ACPI 5.0 support on Linux in San Diego last month during the Linux Plumbers Conference.
The slides for his ACPI Linux presentation have now been uploaded, which are 49 slides that cover low-level power details and the current state of various features being supported within the mainline Linux kernel. Unfortunately, for many of these ACPI items, the Linux state is commonly "not yet implemented", "not supported yet", or are a work in progress. At least though the Linux ACPI state is constantly improving thanks to the continued contributions by Intel and others.
Those wishing to see the ACPI Linux slides from this year's conference can be found here (PDF).
There's also an LPC video of this presentation provided by Ubicast on this page.