From the RFC patch-set for the system device hot-plug framework by Toshi Kani, "This patchset is a prototype of proposed system device hot-plug framework for design review. Unlike other hot-plug environments, such as USB and PCI, there is no common framework for system device hot-plug. Therefore, this patchset is designed to provide a common framework for hot-plugging and online/offline operations of system devices, such as CPU, Memory and Node. While this patchset only supports ACPI-based hot-plug operations, the framework itself is designed to be platform-neural and can support other FW architectures as necessary."
For those interested in this Linux kernel hot-plug framework still being developed and currently up to its second revision, see the mailing list patch series that is currently comprised of 12 patches.
From a separate initiative, back in October of last year was an ACPI-Based System Device Hot-Plugging Framework proposal for the Linux kernel. Last month there was also work towards true CPU hot-plugging support.