Red Hat's David Airlie is reporting on some work done by Matthew Garrett and himself on exploring the situation for those notebooks with combined Intel and NVIDIA graphics. They discovered a WMI driver would need to be written for Linux for passing the ACPI output IDs to the WMI MXDS method for switching the MUX. This at least would support some Intel-NVIDIA notebooks, but evidently Apple notebooks are handled differently without WMI/ACPI, but some Nouveau developers are reverse-engineering that side.
There is yet no working support, but at least some progress is being made in learning how these switch-able notebook graphics processors actually work. Read the blog post here.